Death domain reaper ability
 

Create a new hero with the class Cleric and the death domain.

This has the 1st level ability: Reaper that allows the character to select a necromantic spell from any list and the ability says "NOTE: Once you have selected your necromancy cantrip, you must add it manually to your list of prepared cantrips"

How I do that??

I think in simply select the cantrip in the cleric spell tab but I'm annoyed by the alert status:
Cleric: too many cantrips
Reaper: You have selected the following spell for the Death domain but not added them to your spells know.

No hint on how to do it!!

Thanks (I looked every ware and don't find an exact answer to this)

You should start with 3 cantrips as a basic cleric. Once you've selected the Death Domain, you'll see a dropdown for Reaper in the Class tab to select an appropriate necromancy cantrip. Then, once you've selected that cantrip there, go to the spells tab and add it as an additional cantrip to your list (even if it says "no more cantrips allowed"). You should then have a total of 4 cantrips and no errors.