two things:
first where your setting the variable reference for your widget you need to connect the exe pin (the white one) so the node actually gets called. you can connect it either before or after the add to viewport node, personally id go for before then drag off the return pin and plug that into the add to viewport.
second, on the multiply node the second value should be set to whatever you want the max projectile velocity to be. when i made the example i wasnt sure on a value that would be good so i just put in a random number, you will need to experiment and come up with something good.
that should do it. with all this your projectile initial velocity will be determined by the slider. well it will be between 0 and a set max based on the slider.