RE: How to restrict Assign popup to only allow Teams selection on Assign Button click of Activity Type entity in Model Driven Apps
One approach to achieve this requirement is to customize the Assign button command bar and remove the "User" lookup selection from the Assign popup. Here are the steps to do this:
1. Navigate to the entity's customization page by going to Settings > Customizations > Customize the System.
2. Select the entity for which you want to customize the Assign button popup.
3. Click on "Command Bar" in the left navigation pane.
4. Select the "Assign" button in the "Commands" section.
5. Click on "Edit" to open the command bar editor.
6. In the command bar editor, click on the "User" button and delete it.
7. Save and publish your changes.
With this customization, when you click on the Assign button, the Assign popup will only display the "Assign To" and "Teams" lookup selections, without the "User" lookup selection. This should meet your requirement without the need to build a custom HTML Webresource.
Note that this customization will affect the Assign button for all users of the entity, so make sure it's appropriate for your organization's needs.