How do I calculate this HP for a 3rd party source Owlbear Cub.

I am trying to make the "OwlBear Tamer"  subclass from the source book Tavern Tales by Escape Plan Games.  I have gotten the majority setup in in Beyond.  I am having trouble figuring out the HP formula for this subclass creature.

Level 3 Ranger

Owlbear has CON 14  +2

Hit Points = (Ranger Level) d10 + PB + (Con Bonus x Ranger Level) 

or would someone recommend a better path.   As the subclass levels so does the OwlBear in size and maturity.

Thanks in advance.

Homebrew monster with the details laid out in the class feature. Have the player add the monster as an Extra, adjust as needed.

Thank you both!!!