Integrating two RF application modes

I want to simulate a 3D waveguide geometry using 3D transient wave propagation mode of RF module. The input field at one of the far cross-sections is an eigen-mode of the waveguide.

I create a multiphysics models with both 3D Electromagnetic waves transient wave propagation and 2D perpendicular waves application modes included. The latter is used to define the cross-section of the waveguide which is then simply extruded into a 3D object for 3D transient wave propagation.

After solving the eigen-mode using 2D perpendicular waves analysis, I want to put in the generated field distribution in one cross-sectional boundary of the extruded 3D waveguides. But the problem is how to access the 2D field distribution in this extruded 3D geometry?

Intuitively, I tried defining Extrusion coupling variable in the 2D geometry and selected the desired boundary of 3D geometry as destination. But this doesn't work The variable is not recognized in the scope of 3D geometry.

I will really appreciate if someone could inform me on how to make do with this thing as I am really exhausted searching through manuals in vain. Please let me know if I need to clarify the question or settings more.

Thanks in anticipation.