edit: You learn something new every day 🙂
Do what Bill suggested for a Pan envelope as it’s much easier.
Position your cursor at the beginning of the event.
Click the “Animation Settings” button in the audio track header and select “Show Automation Controls”.
A different symbol should appear in the Pan button.
Slide the Pan button all the way to the left and a red line will appear on the top of the audio track.
Position the cursor at the end of the event and slide the Pan button to the right.
The red line will now drop to the bottom in a curve shape.
This curve can be keyframed like you would a volume envelope by double-clicking on it to add points and dragging them as desired.
You can also right-click on it and choose linear fade, fast fade, etc.
That’s all there is to it.
Remember to add another point at the end of the event, right-click it and choose “Set to Center” or everything else on the track will be panned hard right.