Hi John,
In Access 2013 web app, the table relationship using the "Lookup" field which maintain the relation between form with us automatically. There is no need to use Data Macros, when we open a relative form the relation filed(ClassID) was auto
filled.
Here is an sample figure for your reference:

In addition, you can get more detail about Access 2013 web app developing from link below:
How to: Create and customize a web app in Access
Regards & Fei
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