PostgreSQL user question

I have installed Postgresql on a Windows host.
It works nicely, logging into the admin tool is just fine, but when I attempt to run the command-line maintenance utility, it keeps asking me for the password for an account named after my WINDOWS username, rather than for the ‘postgres’ user.
How do I tell, for instance, dropdb in PostgreSQL that I wish to be prompted for the ‘postgres’ user’s password, and not the password for the ‘mrslant’ user?

It’s been a while since I’ve used Postgres, but according to the Fine Manual you specify a user via “-U username”. You can also use the environment variable PGUSER to specify this by default.