Relative sitemap URL for cache-preload on multisite?

Hi guys,

I am using the latest W3 Cache in combination with WordPress SEO (Yoast) on a wordpress multi-site setup.

Those are my sitemaps generated by yoast:
http://www.mydomain-1.com/sitemap_index.xml
http://www.mydomain-2.com/sitemap_index.xml
http://www.mydomain-3.com/sitemap_index.xml

PROBLEM
Now my problem is that cache-preload does not seem to be working.
I tested it by clearing the W3 Cache, logging out of the panel and calling the home-url of one multi-site.

MY SETTINGS
As the sitemap-url I have entered a relative url “sitemap_index.xml”. I also tried “/sitemap_index.xml” with no success.
The problem seems to be this relative url.

Does anyone know what url to enter in a multisite setup?

Kind regards
Mikey

http://wordpress.org/extend/plugins/w3-total-cache/