How do I enter full addresses into the URL Name field?

I am creating a new site in a SPWI process.  The site name and url are coming from entered field values.

I want the new site to appear as a child of a second level subsite:

   http://siteaddress.com/child/newsite

In the URL Name field I entered:

-  /child/ixml field value]

or

- child/ixml field value]

but in both cases, I receive a "URL contains illegal character '/'" error.

What is the correct syntax to create the site as a subsite of the child site?