I gave my player a Black Bear as their pet/sidekick. They love it so much.

My question is, how do I set the starting Hitpoint for the Black Bear sidekick? Should I:

• just use 19 (3d8 + 6), later +d8 when it level up, OR
• Start with max hit die d8 + Constitution Modifier, later +d8 when it level up

Since it only said 

Hit Points

Whenever the sidekick gains a level, it gains one Hit Die, and its hit point maximum increases. To determine the amount of the increase, roll the Hit Die (the type of die appears in the sidekick's stat block), and add its Constitution modifier. It gains a minimum of 1 hit point per level.

If the sidekick drops to 0 hit points and isn't killed outright, it falls unconscious and subsequently makes death saving throws, just like a player character.

Didn't mention any starting hitpoint

Note: Sorry English is not my primary language, hope you guys understand what I meant

Are we allow to use average hit die when calculating it hit point OR must roll no matter what?

I use average hit point for my player.

BTW, thanks for the information, I really appreciate it