How do you setup a GPO to set Local Admin groups for all workstations

Hello -

I have a root domain with three other child domains with transitive trust from root to all other child domains. Each child domain trust back to root. What I am trying to do is setup all computers within our domains our SCCM CPA account as local admin, again on all systems. I am trying to do this as a GPO. I am running Server 2012 R2 for our DC. Can someone shine some light on how I can successfully do this?

Thank you so much!