How do i make a jump that lets you control how high you go, kinda like the blue soul from undertale

You could do this by allowing the player to increase their jump height like you would use for movement with momentum, until it reaches a cap.

I have made a sample project that shows this linked below.

https://scratch.mit.edu/projects/1362263661/

This is assuming you mostly want to exactly match the blue soul from undertale

My proffered approach would be to have the jump just set a variable (ySpeed) to be something like 15 and change it by -1 every frame. If the ySpeed is greater than 0, you can then check if its still being held up. If its not, just set ySpeed to 0.

This isnt usually a good solution if your making a platformer game that needs to be percise (percision platformer), but i believe its good for replicating the blue soul from undertale.

Another solution (if your making a platformer game) is just to lower gravity while the player is holding jump. That way when they let go, gravity will be stronger so they fall slower. This is a pretty common solution to a problem like this.