Generally speaking of course, your admin url is the url to where you installed OST with an /scp tacked on at the end.
For example:
I've set domain.ext as my business site, and installed OST in a separate web tree. and Setup a hostname called support. That would make my site
support.domain.ext
and the admin url would be
support.domain.ext/scp/
However I could have just as easily setup domain.ext and installed OST in a subdirectory called support. That would make my site:
domain.ext/support/
and the admin url would be
domain.ext/support/scp/