Looks a bit off in this view you shared but I believe the smooth section for the thumb is actually parallel to the base ( but could be slanted if needed)
Here is a bit better view…
Basically 2 cylinders one slightly larger in diameter, and a third cylinder Scalled and rotated for a cutter, which forms the thumb indent done with a Boolean Difference
You just need to tweak the shape of the button, or perhaps it is only a half-cylinder which would look better something like this, then add the 3rd cylinder and the boolean…