Thank you for your answer. First of all, the geometry that I examine in Mechanical only includes solid bodies. However, the force that I want to define is due to oil pressure. Therefore, I want to convert the changing oil pressure as a spring force in the Mechanical Interface. The image I added can help you to get a better understanding. I only define the solid bodies which are shown in the second image.
This is a sample servo valve from an article. When the part is under acceleration, the flapper part shifts according to acceleration direction and it closes the nozzle. The oil pressure rises at that side. I am trying to see the displacements of a similar valve under acceleration. I know the effect of flapper position change on the oil pressure (I made a CFD analysis). It can be called pressure sensitivity. In the structural analysis that I make, I want to define a force on the surface that is shown in the second figure. This force must be a function of the maximum displacement that occurs on the flapper. As the flapper shifts, the force on the spool changes. I have the pressure sensitivity values. Therefore, I know how oil pressure changes according to flapper displacement. The only thing left is defining the representative spring force. The unit is N/mm so it can be called spring force. As the flapper deflects from the default position, the force acting on the spool thanks to oil pressure increases. Therefore, the oil pressure tries to move the spool to its natural position. The result that I want to see is the fact that how much the spool deflects from the default position. Therefore, this hydromechanical effect must be included in the analysis.
The boundary conditions are the red part (flexural sleeve) in the first image is fixed. The spool cannot move in the radial direction but the motion is free in both axial and tangential directions. I will be glad if you can help me.