Inside of the Rubric Manager's Select Tab you can see a button labeled "Select and Assign with Google Classroom"
When you click on this button, if your Google Account is a registered Teacher with Google Classroom, this window will pop up:
The Rubric that you had selected will also be attached onto the bottom of the Google Doc.
For: Select which class to create the assignment for.
Title: You must make a title for the assignment.
Instructions: Here you can write instructions for the assignment.
Due Date: Here you can set the due date of the assignment.
Note: If you enter a due date, you must also add a due time and this must be set for sometime in the future.
Additional Documents: Here you can add additional documents, spreadsheets, files, videos or web links that will be attached to the assignment on google classroom.
Note: There are three options for each attachments:
When you click the Assign button the assignment will automatically be created with the entered settings and attached documents