Here are the steps you need to follow to add a custom assignment category to multiple classes on Microsoft Classroom:

1. Login to your Microsoft Classroom account.
2. Select Classes from the left navigation panel.
3. Choose the multiple classes you want to add the same category to.
4. Click the Manage button on your chosen classes.
5. Select Assignments in the top navigation panel.
6. Click Create category button from the top.
7. Enter a name for the assignment category.
8. Select the display type of the custom category.
9. Configure the grading and points.
10. Once done with the settings, click Save to finish.