Referencing subsite Masterpage in Application page

Hi,

I have a application page, where I have deployed it in the Parent site.

I have a subsite. In the subsite Masterpage gallery, I have a custom Master page.

I want to set the Custom Master page to this application page alone. Subsite is called "Test site" and maser page is "Test.master".

In the Application page, currently I have the below code:

DynamicMasterPageFile="~masterurl/default.master"

How to refere the subsite MasterPage in the above line?

Thanks