That word would be fundatory.
It hasn’t hit the big books (let’s say, the Oxford English Dictionary*) yet, but it can be found in the wild.
Here’s a sentence from The ECoaching Continuum for Educators —
Using Technology to Enrich Professional Development and Improve Student Outcomes (Marcia L. Rock, 2019):
Most teachers, other school professionals, administrators, and eCoaches agree that all should guard against the perils of mandated
fun, also known as “fundatory” activities, and focus instead on
engaging in authentic, meaningful enterprises that help learning team
members work well togwther in better meeting the needs of the whole
Also be sure to check out Urban Dictionary entries for fundatory:
any event or function that is intended to be fun, but is mandatory and will most likely suck
mandatory fun activities that happen at work, which are never actually fun at all
Who knows? Maybe the OED will offer your own sentence as an early example usage someday.
* Therein can be found the “now rare” word fundatory (relating to the founding of something), but that’s not this.