I can only guess it'd take giving them luxury resources for free and chests full of gold, and I'd rather see them smashed under my military machine than do that.
Most of the time if you just give them 1 gold/turn, they take it as a generous trade. And actually sending delegations helps but still just gets drowned out.
They seem a lot less likely to offer cities in peace deals anymore. I've been dominating them and they'll offer me every luxury, and 100% of their gold, but they still won't just give up a city instead. Used to do that all the time in Civ V to slow down a civ I didn't really have time to completely crush yet.