PublishingWebDefinitionCustomMasterPageUrl Field

Specifies a master for publishing pages (Site Master Page). At runtime, the value in this property replaces the '~masterurl/custom.master' token in content pages. Provide a master page file name for master pages located in '_catalogs/masterpages', a web relative master page URL ('SomeLibrary/file.master') or a server relative page url if the master file is located in a parent web.

public PatternString CustomMasterPageUrl