Board Thread:Support Requests - Getting Technical/@comment-4347751-20140814144727/@comment-2170536-20140814174836

Do you need to determine somehow if the field is a valid external link, or is it enough that someone enters anything at all?

If the latter is okay, you can simply use

It would help to see the template, of course, because all that will do is not display the 'Twitter' link when the parameter isn't entered. If it is part of an infobox or table there may need to be additional formatting steps to ensure it looks right.