The interline agreement cannot be 'set' and as mentioned by jasperjugan below, the whole procedure happens at your departure point in the United States. You will have to show the UA agent at check-in your onward itinerary / eTicket so that the agent can tag it all the way to MNL.

Nothing is altered on your ticket nor does it link the tickets. The interline agreement is basically just a reciprocal agreement between two airlines stating that they will transfer baggage between either one so that passengers do not need to pick up their bags at the connection point.