Here are the steps to add an assignment to multiple classes on Microsoft Classroom:

1. Login to Microsoft Classroom using your Microsoft account
2. Select the Classes tab from the top navigation menu
3. On the Classes page, select the three dot menu for the assignment that you want to add to multiple classes
4. From the drop down menu, select Copy To
5. Tick the checkbox next to all classes that you want to add the assignment to
6. Click the Copy button to add the assignment to the selected classes