When suing for peace, there is a control that sets "Monthly Tribute". Unfortunately, a single click of the "+" gives a war value equivalent of over 400. This is an impossible agreement to get. I've tried to shift, alt, and ctrl with the click, hoping that it might select a lower value, but it is no help.

It seems to me that there must be some way to accomplish this since: a) there are missions that require you to receive tribute, and b) in the events description when other countries' foreign wars end, sometimes there are tiny tribute awards.

So what am I missing here?

IIRC,

If the Warscore shoots up to impossibly high ratings due to demanding monthly tribute, it is due to that faction having a lower monthly income than that demanded. So they can't even afford it.

If the minimum amount you can ask for is > 400, then nobody can afford it and the feature is useless.

-edit- When AI powers offer peace they can offer or demand down to fractions of a gold.