How do you change antifreeze in a 1300 vtx?

I changed the antifreeze in my bike and according to the manual, it says to add only through the port under the seat. That port goes to the overflow bottle underneath. There seems to be no siphon to draw it into the radiator. I know this because I rode the bike until the coolant light came on. Checked the bottle and it was still loaded with antifreeze. Only other was I can think to add it is to remove the hose about the radiator and add directly to the radiator. In the meantime I have more that I need in the over flow bottle. It does drain out to the ground if you try to add more than it has capacity for.