Master page for a Web form can always be set manually in the source code, but here is an option to select the master page while create the webform using the 'Select a Master Page' dialog.
The path to select a Master page using the 'Select a master page' dialog is different for a Web application project than for a Web site.
For a Web Application project:
For a Web site project:
Hope this helps,
Deepak Verma SDET | Visual Web Developer