Extra Field when posting New Thread

[BasilFawlty]

This may exist, but I did several searches and didn't see anything. What I would like to do is have an extra field on the "newthread" forum, but only for certain forums I designate. I have a few forums that are "Classifieds" forum and I'd like to have an extra field, probably just below the Title field, called "Ad Link" (or similar). Then, if someone is posting a classified ad and they have the item also on Ebay or Craigslist, for example, they could paste their link into that field (optionally).

Then, when they save their post, "IF" that field is not empty, that link would get parsed and posted at the end of their regular text. The mod would also look at the first X string of characters in the URL they entered and, for example, if the string contains "Ebay" then the link might look like this:

EBay