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:

1. Right click solution explore. Select Add and then select New Item
   
   
   
   
2. From the Add new Item dialog select Web content form and click on Add button.
   
   
   
3. Select a Master Page dialog would appear. Navigate thru the project folders to select the Master page & click on OK button. The aspx added would now be using the selected master page.
   
   

 
For a Web site project:

1. While adding a Webform, check the select master page checkbox and click on the Add button.
   
   
   
2. Select a Master Page dialog would appear. Navigate thru the project folders to select the Master page & click on OK button. The aspx added would now be using the selected master page.
   
   
   

Hope this helps,

Deepak Verma
SDET | Visual Web Developer