The relevant input field in the ZoneHVAC:LowTemperatureRadiant:VariableFlow zone equipment object that will link the system to the internal source construction surface is 'Radiant Surface Type'. The choice in that field will tell OpenStudio to create a ZoneHVAC:LowTemperatureRadiant:SurfaceGroup object on translation to idf that includes the surfaces in the zone of the selected type that have internal source constructions, and reference that object in the LowTemperatureRadiant object's !- Surface Name or Radiant Surface Group Name field.

Note: A while back I documented an issue with this translation that I'm not sure was addressed by the developers, so be aware. It shouldn't be an issue if you don't have any surfaces defined with internal source constructions in the space that you _don't_ want linked to the system.

You're correct that if you leave the loop connection blank, you will run into an error. As usual with these situations, you can link it to a cooling loop, but force cooling to never be activated by hard-sizing the 'Maximum Cold Water Flow' field to zero, or defining a Cooling Control Temperature Schedule such that cooling is never activated.