Once you've signed up as a free agent, you've done all that you need to do. The next steps are up to any captains that have signed up a team in that league.
Captains have access to everyone who has signed up as a Free Agent for that league.
They do not have your contact information. If they happen to need players to fill out their team, they will contact you through the system to offer you a spot.
Any payment for your league will be handled between you and the captain who offers you a spot on their team.
Underdog has no involvement in the process of placing you on a team or collecting your payment.