How can I share a PowerPoint in SharePoint so that it opens automatically in presentation mode?

Sophia's reply is correct. The short, not-sweet version is: You can't.

This is a serious problem for many people. Please report it to Microsoft as a feature request. Use the Feedback feature in PowerPoint to request that this be fixed. That's the best way to tell the developers that they fell down on the job and it needs improvement.

Dear Alison,

Good day!!

Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.

I understand your concern and to my knowledge, the PPSX formatted presentation will be opened as a normal presentation when you are viewing in the PowerPoint for the web, where we need to manually click on the Slide Show and there is no setting or feature in the PowerPoint for the web to play the presentation automatically in the presentation mode.

I agree with you having the ability to view the presentation automatically in the slide show mode when viewing in the PowerPoint for the web will benefit the users and suggest you to add your valuable idea as a feedback in the PowerPoint · Community (microsoft.com) which is the best place to share our ideas and improve the Microsoft PowerPoint product.

Moreover, the alternative way is to save a copy of PPSX to the local device or sending the PPSX file as an email attachment to the users via Outlook or any email app, so that they can save to their device and open it in the slide show.

Appreciate your patience and understanding. Have a good day!!

Best Regards,

Sophia