F1 2018

During career mode, you will get an option to negotiate a new contract. One of the options you can choose is to be first driver of the team. For me, this happened while winning the first 4 races in career mode (so after Baku). Originally, Mercedes declined by first offer but after the second offer, in which I just said I will finish 3rd for Quali and the race, they accepted it.

Achievement unlocked after I continued for to the next weekend.

You dont need to change teams for this. I won the first 4 races with Ferrari in career mode. After Baku you are given the opportunity to renegotiate your contract or select a new team. You have 4 areas of negotiation available. Personally I choose (1) to qualify at least 2nd, (2) to finish 3rd, (3) to be number one driver, and (4) I set the Team Goals to hard. Ferrari accepted my contract and while loading the next race the achievement popped along with another for being the teams 1st driver.