You can create the object with a microflow in stead of the default new button.
Then in the microflow retrieve your default value for the reference selector and in the create object action set the association to this default value (object).
Check the microflow that returns the list of selectable objects and see if the default value you want to use is in the list returned by that microflow. If not, that is your problem.
Also, I have a question. You said you set a default value for the reference in the domain model. As far as I know, there is not a way to do this. You can set default values for attributes, but not references. Could you provide more information about how you set a default value for the association in the domain model?
Hope that helps,