I never use a tensioner on the motor chain.
The reason everyone does is because the kit comes with one and suggests you use it on the chain that came with the kit as that is what you are already working on.
But think about it.
A tensioner is not some crucial part that needs to be on, it is a fix for a simple chain length mismatch problem.
Fix the actual problem and you don't need a fix for the symptom.
This is the Grubee GT1 bike and GT2B motor bike I am almost done with.
I used a 1/2 link on the drive chain and got to only about 8mm to long.
Then I put a flat shim between the motor and the seat post mounting block to take out the motor chain slack.
For shims I just use what I have plenty of, the slot covers on computer cases. After 27 years of building computers I have a box full.
I just fold it in half and put some thin packaging cardboard pieces in between until I have the thickness I want and put in it. As a bonus the little lip on it keeps it from falling through;-}
But even if you don't want to mess with that, at least consider putting the tensioner on the pedal side.
You don't need to use a big ugly heavy duty bolt on tensioner either.
Your pedal chain hardly gets any use and simple little bicycle tensioner will work fine.
If tensioners were a good thing on a motor driven chain, all motorcycles would have them right?
It is a little more work, but in my book worth every minute of it to avoid an un-needed moving part in the main drive so I do that on everything I build.