I can’t figure it out😭
I’m wanting to make one similar to the way super smash bros works.
help me please
Your health is most likely stored in a number, be it within the health behavior or a variable. This number can ge obtained through an expression. Any number field, like the force strength, can use a mathematic formula and expressions. What you want is to increase the force strength, aka how many pixels per seconds the force moves the player object.
To do so, assuming your health is a number that goes up like in smash, you can simply use the number. If it is stored in a variable “Health” on the object, you could use the expression PlayerObject.Variable(Health) as the force strength. It likely won’t send the player very far, though: 50 health would only move the player by 50 pixels 
You can multiplay the health by a constant to make the player get moved more. You might also want to have some knockback even at 0 health, and for that you can add a baseline to the product: PlayerObject.Variable(Health) * 4 + 50
You might recognize a linear equation here (m * x + b). Of course, if it fits your game better, you could chose an exponential one instead to have the knockback grow exponentially with damage: pow(PlayerObject.Variable(Health), 2) + 50