a field to accomplish this?

I have examined all the available field types and am not sure which, if any, can accomplish this. I want to enable people to post a url to another website but force them to only be able to do so to a pre selected list of sites.. Example: a link to their youtube channel. They would only be able to place a link with the prefix of https://youtube.com/{this is where they can insert their link}. That way rough users can't insert spam links to untrusted sites.

I had hoped a custom code field existed in easysocial but I can't find one and I could not find anything in apps to accomplish it with.