How to create a Unity ShadowMapTexture

HI, currently I am working on shadow mapping, and trying to use and modify a few techniques from the standard shader Unity uses. Everything works finde, except, that I want to create the shadow Map Texture myself. I am able to create one, but I can`t use the standard shader techs with my own shadow map. Anyone knows how I can create exactly the same shadow map texture Unity creates? Can`t find anything in the shader files.