Hi,
there are some possibilities:
Firstly: does your outflow occur mainly at the river channel ?
If yes, you can manually calculate a q-H-relation with any 1D HN - software otherwise you can use the so called "Normalabfluss" (normal discharge) after GAUCKLER, MANNING, STRICKLER. This is usually a good assumption.
If no: you should calibrate your outflow boundary due to observations like flooding areas oder other flood land marks.
Generally: you should extend your model to the downstream so far that the downstream boundary does never effect the water levels in your area of interest. This could be very easily done with FM, because you can increase the element sizes off the AOI very strong and the extension of the model to the safe side dows not cost very many nodes and elements.
With this approaches, a Q-h-relation is in my opinion the best controllable and most stable boundary condition.
You define the Q-h-relation as rating curve. You will find the explanation in the MIKE21FM documentation.
Hope that helps,
Christian