You need to give the freelancer the job. You need to open a contract, either hourly or fixed-price, and pay the freelancer for working for you. Here is a guide to get you started.
If you have already "tested" them, then they must have applied for the job, so you will see a HIRE FREELANCER button. Press that, get the ball rolling.
I don't know why but if I go in contact on the voice of this coypwriter I have the button with Next Millestone written and I don't know why but I must have done something wrong
I checked this for you and looks like the contract you're referring to is still active. If you discussed this with the freelancer you can go ahead and add the new/next milestone. You and your freelancer can continue working on your existing contract, while you also have an option to close the existing contract and rehire your freelancer for the new contract.