Hi @Rebecca O'Donnell,
Thank you for posting in this community.
Indeed, we cannot search a Viva Engage feed onto a Conversations web part based on hashtags. This is designed by Conversations web part.
You can use SPFx to create a new Conversations web part, but this requires you to know enough about Conversations web part and have some coding skills. You can refer to this article to start your custom web parts: Build your first SharePoint client-side web part.We are always looking for ways to improve SharePoint, and your feedback and suggestions are very valuable.
We suggest that you can make suggestions on this SharePoint Feedback portal for embedding a Viva Engage feed onto a SharePoint page based on hashtags. Your contribution will be highly appreciated.
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