You add a key frame to any object in a Tween timeline as follows:
Select the object to add a keyframe to by either by clicking on the object in the main editing window or by clicking on the name of the object in the tween timeline list at the left of the tween timeline window.
Right-click on the relevant frame marker by clicking between the tick marks on the ruler at the top of the timeline
Select Set Keyframe from the menu which appears.
The keyframe will appear as a black rectangle in the bar relating to the relevant object.
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