I want to mount a bathroom bar light above the vanity in my bathroom.

This is old work. I already have the electrical work done and the wires are
ready the issue is that I don't have a junction box in the wall above the
mirror.

I was going to get an old work ceiling box from home depot and mount it in
the wall.

Is this the correct item to get?

Also how much weight can it support? I guess there will be some contact
force with the wall that will aid in the support.