User define force function of variable "x"

For the implicit structural analysis used by Creo you cannot define a force as a function of an analysis dependent result/measure, only time. However, you can make an independent parameter have control over the value of the force in the force definition.  The independent variable (parameter) is stepped using a sensitivity study, or optimized as a variable in optimization.

Using Parameter As Independent Variable

Actually time is just an independent variable for CREO, it is not really time, so there is no reason not use use it for X instead.  This is because CREO is implicit and does not have dynamic effects.  Try it that way and you should get the same result as my example above.