I use a Visual Studio 2019 CE and a TFS 2018 Update 3.
I installed the Process Template Editor from the Marketplace: download site
Here you can edit the WITs like in the old editor. In the 'Workflow' tab I can edit and modify everything, I can even add new 'State' items. But It seems I cannot add a new 'Transition Link'. Has someone an idea how to make that work? Maybe I missed something or is this a bug?
Outside of the Work Item Types (WIT) pane there is a Toolbox
tab that opens up a WITDesigner control panel.
To make a transition link you must first select Transition Link in the control panel. You'll then move into the WIT pane where you must select the starting state, then select the state that's being transitioned to. Dragging the transition link is disabled.
In other words, to add transition link, there should be a starting state and target state. Hope this helps.