How to reference a master page other than the "default" in custom Application Page

I have a custom Application Page that's using DynamicMasterPageFile="~masterurl/default.master" to get the associated master page. I'd like for this Application Page (and others) to use my custom Master Page that I created using the Design Manager. This master page is stored with the others at [MappedDrive]:/_catalogs/masterpage/custom.master

In Site Settings, I can change the System Master Page to use my custom master page, which in turn will cause my Application Page to use it.

However, I don't want all the administrative pages of SharePoint to have my custom site master page, I'd rather have them use seattle to have that native SharePoint feel. I only want the specific application pages that I'm creating to use my custom master page. How can I set the DynamicMasterPageFile attribute to reference my custom master page?

SharePoint 2013 On-premise