Get to know her and bring up the subject that she has a webcam. Ask to chat, start out just a chat, call her prettya nd compliment her lots, say you have a boner from her, ask if she wants to see it. You can handle it from there.
Its kinda difficult to convince a girl to have webcam sex , cause girls on webcam are more shy than men. If this girls is your girlfriend is a matter of time to convince her, if she finds it...