With TrackX you don't keyframe specific parameters, you keyframe the mask as a whole.
First, you MUST click the Key icon in the left hand HUD to turn on the keyframe abilities. A set of keyframes tools will then show in the lower portion of the Viewer pane.
Then you draw a mask shape on your first frame. The mask control points will be green, so grab one of the mask control points, nudge it slightly so they all turn yellow. Yellow means you just created a keyframe for the mask.
Then go to the frame that the movement should stop on, and manually drag the tracker (middle dot) and adjust individual mask control points as needed.