Hi,
Takes allot of time to develop the corpus spongiusm [The middle chamber surrounding the uretha] and for me it came gradually from doing standard constrictors.
I think that lateral jelqs would work for the area too but not the normal method of doing jelqs as you will need to place stress on that middle chamber and with the normal jelq you only hit the sides really, so you can reverse the hand position so its lateral and jelqing topside > bottomside instead.
From my years of pe I cant say I've heard an exercise that was made for just hitting the CS as its in such a tricky spot, but do try the standard constrictors if you have the experience already or lateral jelqs which are as I say just the same as normal jelqs but with the hands jelqing the top & bottom instead of the sides.