Create a section of 3D geometry that you want to turn into a displacement map, not too large a section, treat it as a tile in the same way that you would create a conventional pattern image.
This example is based on a diagonally arrayed perforated sheet from UK manufacturer Ash & Lacy:
Select the geometry, then in the Material Editor > New Material From Geometry:
Follow the prompts to create the material (but check that the size setting on the material preview is a sensible one for the size of the source geometry, the size needs to be larger than your geometry but not too large that you can't see the features). The material shown below is based on a this model:
The process generates two image files per material.
These files are created with the name of the source design file, so will need to be renamed and reattached to the material definition to make any sense.
The new material will be called 'Displacement Material n' and will have matching HDR files where n is the same:
renaming your material won't change the name of these files, it has to be done manually.
Each pair of depth and opacity HDR files created in the same folder as the source dgn will need to be moved to your project materials location to share between files: ...\Resources\materials\bump.
So to recap the steps are:1. create material.2. Rename material.3. Rename 2 x HDR files and reattach to the material4. Move HDR files to your project resources\materials\bump folder as appropriate.