We have asked Apple to open them up for 3rd party developers, if they do it would be trivial for us to support. Until that time, and if you would like to see this happen to, make sure you file a feedback item for Apple at:
https://feedbackassistant.apple.com
You can file a feedback request for the Reminders app to support these kind of “ app links ” so 3rd party apps can create them through “ EventKit ”.
If you do make sure you mention that the makers of Agenda app have filed their request under feedback issue FB6434150
The more feedback they receive the better, and the higher the chance they will add this ability.