There are some considerations when adding .NET Connections in Revit.
In order for the Revit families of elements for the input set for the connection to be compatible with AS .NET Connections, their family symbol has to have the Structural_Section_Name_Key parameter and the StructuralFamilyNameKey property set in accordance with the table RevitASProfileConversion table from AstorProfiles.mdb. These two (the parameter and the property) cannot be set from the user interface. The Advance Steel connections currently work on the Revit structural beams and structural columns families; these families have already been preconfigured starting with Revit 2016.
If the user selects input elements whose families do not have the Structural_Section_Name_Key parameter or the StructuralFamilyNameKey defined, the connection creation will fail.
The connection parameters are saved in Revit's extensible storage. The extensible storage can be accessed with the Revit API.