I've seen posts that describe how to set postgresql server to connect to your own user but I could not find any posts that describe how to make psql connect to the default postgres user instead by default. In my case, right after installation when I open up psql, it connects to the username with the name same as my windows user instead of the postgres user that I want it to connect to.
As documented in the manual you can define an environment variable PGUSER which is used instead of your Windows user as the default user for all Postgres command line tools.