Help with custom .fgd

I’m trying to create a custom fgd for a gamemode I’m working on. I would like to be able to use the same auto-complete function found in entities like path_track or keyframe_rope where you can shift+drag one of these entities and have the targetname automatically fill in. I tried copying and pasting the path_track code from the base.fgd and slightly adjusting it to my needs, yet it still will not act the same. However, if I fill in the targetname manually, a line connecting the two entities will appear.

Does anyone know how I can add this auto-complete functionality to my custom hammer entity?

Here’s what I have for the entity so far in case anyone needs to see it:

@PointClass base(Targetname, Parentname, Angles) cylinder(255 255 255, targetname, target, radius, targetname, targetname, radius) color(50 125 255) size(32 32 32) = custom_ent :
	"This is a custom point entity for hammer."
	spawnflags(Flags) =
		1 : "Don't use this as an available location" : 0
	target(target_destination) : "Next Space" : : "The next space to go to."
	skin(choices) : "Type" : 0 = 
		0 : "Blue"
		1 : "Red"
		2 : "Green"
        //Once a model has been finished I will be using it in place of the small white box I'm using now, hence the skin choices parameter