When you select the "Other" format have you then entered some characters in to the auto complete (i.e. the first few letters of your custom format? It should appear in the list of other formats under this autocomplete.

If it dosn't then check that you have associated the skin with the application on the application rule.  You can also set an override skin in your operator preferences in the Designer Studio so also set it here.

The value you specifiy in the autocomplete when selecing "other" is not validated anyway, so you can simply enter "StandardWorkMgr" and save the section.  If you application is using the skin with that format then it will still work.