Hi Mikesira,
Assuming that you have multiple materials setup for your mesh and the window material is it’s own material you will need to set the following things.
Material:
Blend Mode: Translucent
Two-Sided: True
Lighting Mode: Surface Translucency Volume
Then for your mesh placed in the level select it. In it’s Details Panel make sure that Volumetric Translucent Shadow is set to True.
This will cast a shadow from a translucent material, however the more opaque the material the more you’ll notice artifacts on the mesh itself.
Here is an example with a simple cube mesh and basic translucent material.
You may also notice that there is transmission of light (ie. color of the translucent object does not cast to the ground). This is not implemented currently and can only be done by using only static lighting
I hope this helps.