You are RARELY (almost never) hired into a base of your choice for an airline. Typically sometime during training the company will have a list of vacancies meaning simply what they are short on, at that time you will list your preferences and based on seniority (you are all the same seniority date in class so it usually goes on age or sometimes last four of SSN) you will be assigned.

Typically nothing you do will have any preference on what you get to fly or where you get to go although you can usually have a pretty good idea based on company intel or the company may straight out tell you that they ONLY need dash 8 pilots in Philly at the time you are in class so thats where you will go.

It also goes in cycles throughout the year where for a while at CAL alot of guys went straight into the 777 and then a few classes later the junior guys would go 737 and back and forth.

What you THEN do to get what you want is your first day on the job when you are able you put in a bid request into the base and equipment of your choice and just wait until your seniority can hold it and your seatlock (if applicable) is up.

Long story short...there is NOTHING you can do to influence anyones decision regarding which equipment you fly. Airlines are absolutely 100% driven by seniority and the only way into a 777 over a 737 is 1. the company has a vacancy for that airframe and 2. you having the seniority to be awarded said vacancy.