Which are my options to find trip companions online?

I found one that might work, literally only on the past weekend. So good timing, potentially. And I'm aware of a more adventurous companion one too.

Kicktraveler - the American spelling of traveler, but I'll let that pass - allows you to join or design trips that you want someone to come along with. It's like crowdfunding your travels - say you want to join a trip in Egypt, but want others. Find someone who already has planned a trip who needs more in their group and join them, or start your own one, show an itinerary, costs, and then wait for the travel buddies to join in.

There are understandably a few commercial tours who have cleverly marketed theirs on there as well, but certainly it's a way to find companions for a specific trip.

On the other hand, if you're looking to do a more adventurous trip - bush hiking through the Amazon or a crazy adventure into the Antarctic, something like Explorers Connect might be more suitable - there are always some amazing adventures looking for trip partners (often with specific skills - doctors, drivers, photographers), or you can propose one yourself.