Is Custom Table Not showing in Activity Regarding?

Implementation Steps:

1. Navigate to https://make.poweapps.com

2. Click Gear Icon at the top

3. Click Advance Settings

4. Click Customization

5. Click Customize the System

6. Click your Custom Table

7. Make Sure you have enabled Checkbox for Activities & Enable for Unified Client

8. Once Done click Save and Publish All Customizations

That's it 🙂