You just need to type :
in the form field.
The error you're getting is what I get if I type the whole URL into the form field eg.
Code:
http://www.asktravelinsurance.com/google-sitemap.cfm
Google knows your file is going to be at or in a directory of
asktravelinsurance.com so doesn't let you specify or change the domain.
Hopefully that is that simple, and you'll at least get a bit further.