System configuration GDM entry for home-manager installed WM?

I’m trying to add login entries to GDM to boot into Home Manager managed river.

I currently have:

		displayManager.session = [{
			manage = "window";
			name = "river";
			start = ''
				river &

But it just throws me back to gdm when I try to login.

Is this even possible or do I have to put it in my system config?

(I want it in my home config so it follows my home-manager flake because river config doesn’t seem stable enough to keep configuration on a separate pin from the installed version yet)