Hello tristeng,
If you are calling the script from the command line via -r argument, you should have the related geoid file in /Metashape Pro/geoids/ directory whereas the coordinate system can be assigned from the WKT line, for example:
wkt = '...' #just as a string with the coordinate system definition
crs = Metashape.CoordinateSystem(wkt)
In case you are using the stand-alone Python module, you should initialize the path to the geoid file in the beginning of the script (before the related coordinate system is used) by calling Metashape.CoordinateSystem.addGeoid(path) method.