For Suryavarman I campaign

one level that you can either defeat 4 of 5 enemies or build wonder, in the tip it said you can “maybe” persuade Vietnam(yellow) to join you to attack north Cham(green), but how? yellow is an ally at the beginning and how to ask them to join the war?

i saw in some game video, the yellow asks for 1000 gold to join the fight but how to trigger that?