I have zero experience with Muse - or OAM files - but no one else has answered so I am going to throw out a possibility (based on the threads below) is that You are going to need to hand code it in Muse.
Post edit note: I have just done some googling and it does not appear that You can add your own code in Muse - if so it would have to be done in an HTML export edit, which would have to be re-edited with every export. Is this assessment accurate?
Also another edit at the bottom of this post.
A response from @Jonathan (1st line is what I’m referencing):
Note: This thread also has a solution from @MarkHunte that may be pertain to your situation.
Further Edit: I looked up “Viewport Actions” in Hype’s documentation and found this morsel:
Note: If your document is in an iFrame these actions will fire as you scroll the iFrame, not the main frame. If your iFrame is not scrollable and the element is visible when the iFrame loads, the action will fire immediately, even if the iFrame itself is not currently visible in the browser.
This would explain why “The OAM simply starts with the final frame.”