When renting a car, how to get the exact chosen model and not the similar?

If you read more carefully when you booked, you would almost certainly have seen the like "VW Polo or similar" with the "or similar" often in much smaller text.

As for what you can do, you can book with Hertz and pay extra to use their "book this exact model" option. There may be other hire companies offering this.

In general car hire companies will advertise more desirable cars, then give you a Vauxhall/Opel. My experience is that at least in the large hatchback or smaller categories, you are very unlikley to get a VW - the cheaper ranges in the VAG group (Seat and Skoda) sometimes, but rarely a VW, never something premium like BMW or Audi without paying extra. However some companies do allow a "special requests" or similar text box when booking. I've found that expressly requesting not a Vauxhall works to avoid being lumped with corsas.

Overall, it's good planning when booking a hire car to ensure that the "worst" car in that category would suit your needs. I've had trouble in the past with needing storage space, seeing "Polo or similar" and thinking that would be fine, to be given a smaller Opel or Citroen. I've learned to book for larger when needing the space etc.

Make sure that the contract you sign either has a tick box for this exact car, and also put that in the comments box. If booking by telephone, make sure that the operator understands that you want that make and model, and will accept no substitutes.

I don't know about the OP, but we quite often rent cars for a weekend trip to see if we want to go out and buy that particular car, rather than just having a short test drive from the dealership. So if I want to test drive a Subaru Outback, don't bother trying to give me some sort of Chevrolet, or even a Subaru Forester; I need an Outback, and no other vehicle, regardless of size, will suffice. It does restrict our choices to renting from the dealer (some do), or the larger rental companies, but we get what we need.