People don't have time to make tutorials with pictures for every question that people have, unfortunately. You are going to have to work some elements out yourself.
You want to make a jump where the height jumped is dependent on how long you have been holding the key down.
This is done in two stages.
Stage 1: find out how long the player has been holding the button down.
Stage 2: Use that to determine how high the jump should be.
So that sounds easier now. To do stage 1, you need to use the System.time expression when the key is pressed - store it in a variable. When the key is released you can use the System.time expression again to get the new time, subtract the old time (which you stored in a variable) to get the length of time the key was held.
To do stage 2 depends on how you are doing a jump. Are you using the platform behaviour for example. Anyway - hopefully you will know how to make a jump. All you have to do is use set the value of the jump height using the value you obtained, probably with a multiplier, probably also clamped to sensible values.
Hope this helps.