How do I do this? :P

[CampinCarl]

I have a custom profile field, and people keep incorrectly entering the information that needs to go in it, so i just need to know how to make it so the information they enter has to begin with the correct phrase
What I need it for: I have a custom field for someones steam ID, so they can enter their steam id, and they keep entering their steam name, so intstead of getting STEAM:0:0:1234567, im gettin stuff like "johnfrink", so i need to make it so whatever you enter has to start with STEAM:, :P

thanks