In Workbench, you can add a Direct Optimization component to the Static Structural analysis and let the optimizer adjust the force until the displacement is exactly 50 microns.

I hope an expert in APDL can answer the question for how to use APDL code to do the same.

How linear is the force-deflection data?  If it is basically linear, then just apply the same force to all designs and use the deformation of the top as the response. Even better, divide the force by the deformation to put the response in terms of stiffness K in N/micron.