Adding property to master page in webforms from external page.

Hello!
 
I'm trying to add a property to a masterpage from an external page of the type ContentArea.
Normally I'd be able to reference the property from the model, but I can't do that now(that I know of). I've tried creating a property in the codebehind for the master page and then assigning the property from a startpage reference(in the onload method), but I can't find any of the assembly classes that can hold the property...(in either system.web.ui.webcontrols or episerver.web.webcontrols).

I'm quite a junior developer so I appologize if some of my terms are a bit off :) if you need clarification to understand the problem, please ask away!

Cheers!