In case you were not using this command before, it is still available through the .Arma3Profile file. Adding keyForceCommandingMode[]={57}; will enable it on the spacebar.
Open a new editor session and place the player and preview.
Press commanding mode (^space) you will notice you have access to supports.
Press 'Select all' nothing, no menu at all as you have no subordinates.
Go back to the editor and place a subordinate for the player and preview.
Press 'Select all', you now have access to supports.
Press Space and you will notice it has exactly the same functionality other than this one behaviour.