Restrict fromNode ports to create a link

In my software, for each node we have some toNode and fromNode ports. Right now, fromNode is able to create links. It works fine as the direction is towards fromNode port and it is connectng to toNode ports only. But I want only toNode port can create an arrow.

If you only want links to be drawn starting at the to port, maybe you want to set LinkingTool.direction to go.LinkingTool.BackwardsOnly.