Hello,
i have been holding my french bow with my thumb between the top of the frog and the bottom. (that little sideways U in the frog) and i am seeing that some people put their thumbs on the stick
what is the correct way to hold the bow?
thanks

The thumb should be on the stick.

It can be held either way. Put your thumb where your able to control the stick best and have the greatest power. I often put my thumb in the spot you're talking about and know others who do too.