I want to creat a linked form that will automatically enter an ID number in the "subform" without actually making it a sub form. Here is what I have:

The master form has a field (autonumber) [Estimating ID] for each individual project and I want to create multiple hardware groups for that project. The tables are linked via the [Estimating ID] but I want to be able to click on a button that opens the Hardware Group form (preferably in a pop-up form) and automatically enter the [Estimating ID] into the Hardware Group table for each group that I create.

How do I do this?

You will need to write code to pass the EstimatingID field value to the other form.

One way is thru the OpenArgs of the DoCmd.OpenForm action.

There is information in the help files.