You'd use arctan.
Tan(angle) = opposite/adjacent
angle = atan(opposite/adjacent)
Opposite = difference in Y position (touch position - stick position)
Adjacent = difference in X position (touch position - stick position)
However that only works when the angle is -90<angle<90. That's when the Touch position is to the right of (greater than) the stick position. Otherwise, using a If Then Else block, subtract 180.
I've an example on Hopscotch. Search for Point Towards by ThinBuffaloSr. I think the fūll name was Point Towards (using inverse tangent)