Visiting a concert in japan

SHORT ANSWER
Without help from a friend in Japan or using a proxy, it will be difficult to get concert tickets, which are hard enough to get in Japan.

LONG ANSWER
Currently, there are no general sales tickets available, and they have yet to announce the date.

On the site you posted, the following was posted along with the concert schedule:

???0????????????????FC??????????????????????????? ?????2019/4/7??? 0:00 ? 2019/4/21??? 23:59????
Translation: Advance sales raffle applications for are accepted through the official fan club from 4/7-4/21

It is expected that they will announce the general sales date after the results of the fan club raffle. Some events announce all the dates at the start, some (like this one) announce only after the last wave of sales is done. The official fan site lists the result period as 4/27-5/1, and at the same time I assume information about general sales will be announced as well.

Some things you should know:

1) Raffle
By the looks of it, I'm sure the general sales will also be a raffle. This is common in Japan for popular events. This means that you will not be guaranteed a ticket until late in your planning, so I suggest starting making alternate plans if this falls through. Depending on the concert, there are also "Live Viewings", where the concert is satellite broadcasted to select movie theaters. I couldn't find any information regarding that for this concert, but sometimes live viewings are announced fairly late. Also there are slim chances of onsite tickets, but looking at the size of the venue, it seems unlikely.

2) Accounts
They are using e+ for the fan club tickets, so I am assuming the same for general sales. You will need an e+ account, which means you need a Japanese phone number to verify the account. You will need to ask a friend in Japan, or use a proxy service to help you buy the tickets (never used any before, so I can't attest to their trustworthiness).

3) Payment
If you do manage to create a valid account with a friend's help, note that you have to have more help when paying. Many ticket sites, including e+, do not accept foreign credit cards. The best way is to have a friend pay through the convenience store (e+ usually offers 7-11 or Family Mart), with a nominal transaction fee. Note that payment option MUST be selected when initially submitting, which means making the wrong choice (i.e., credit card) will result in forfeiting the ticket. And make sure to check the results of the raffle promptly, as the payment window for concert tickets is usually short (3-5 days).
It would also be a good idea to choose to receive the tickets at the convenience store as well, so you can just pick it up when you arrive (usually you can print 1-2 weeks ahead of the event).

4) Number of Tickets
You are limited in the amount of tickets you can purchase. Considering the concert is in Tokyo and at a smaller venue, you will most likely only be limited to 2 tickets. If you need more, your friends will have to apply as well.

5) Name
Your name is printed on the tickets. If you do get the tickets, remember to bring your passport. The operators may or may not choose to check ID, but there's no point risking it. So whether making an e+ account or using a proxy, make sure your real name (preferably exactly your passport) is used. Also another reason against buying scalped tickets, unless the ticket holder is going in with you.

HOW TO APPLY
I haven't really shown how to apply for the tickets. So let's assume you have a legit account and that it's on e+. It's going to require a lot of Japanese navigating, and you have a limited time before the site times out and you'll have to start from the beginning.

1) Information
I'm assuming you have made it to the e+ page for the Lisa concert of choice. From there, you will select the basic information of the concert: ???? (concert time), ?? (seat type), and ?? (amount of tickets). A lot of concerts will only have one concert time, though smaller ones will have two in one day, and you'll usually pick which one here. For seat types, again, usually it's only one type. Some have premier seat, and some have standing-only seats. Once you're done click on ???? to go to the login screen. Make sure to get the password right, as three fails will lock your account for a while.

2) Receiving and Payment
Next screen will be about receiving and payment, in that order. ???? is the "Method of Receiving" section and "????" is the "Method of Payment" section. Note that the choices available vary from event to event.

3) Final Confirmation
This is the confirmation screen. If everything is well, press the pink button ??????. Note that some events require you to check waiver clause first, usually pretty close above the button.

Note there are a few guides out there on the internet that provide some useful information. I found this one or this one after a quick google search.

With a bit of random clicking around, I found this site https://eplus.jp/sf/live/lisasp

However, googletranslate suggests that it might be sold out, and that's a resale site. To find that page, starting from your Disk Garage page, I clicked on

1. ?? at bottom
2. purple button on end of row
3. e+(?????)

BTW, we have a Japanese learners stack; they might be able to help ( https://japanese.stackexchange.com/ )