I have created a new Site Master page in my site. I want to assign the master page to just one single sub-site page. I've attempted to change the "MasterPageFile" but was unsuccessful (I know this is set as a referential). The Site Master Page is in both my subsite and master site's master page's folder. Is there a specific URL path to use to change the MasterPageFile at runtime?
You can refer your master page in different ways as shown below:
Site Collection-Relative Master Page: ~sitecollection/_catalogs/masterpage/custom.master
Site-Relative Master Page: ~site/_catalogs/masterpage/custom.master
Relative path are based on where the page resides: "../_catalogs/masterpage/custom.master"
Hope it helps. Thanks.