Hello RalfH,
To generate the orthomosaic from the single image you need to define the exterior and interior orientation parameters for the camera. So you should have XYZ coordinates of the camera center, yaw, pitch, roll or omega, phi, kappa angles and calibration parameters.
Providing that you specify these parameters and import the surface (DEM or mesh) in the same coordinate space, you would be able to generate the orthophoto.
Unfortunately, currently PhotoScan doesn't support image georeferencing based of the control points (like some GIS application do).