I want to put a textbox on a form that will take user input (a postal code) and put it into a table. I'm not sure how to do this. Is it even possible, and if so, any suggestions?

Make a form with a textbox bound to your table.

How do I bind a textbox to my table?

First create your form and choose the record source of the form. Next create a textbox and then make its control source the column of your table.

If I already have a form going, do I need to create a subform? I haven't used a subform before.

No, have you set the record source of your form?

No, I'm using the form to pull data from another database, then I'm making a new table where the user inputs data that I planned to append to the pulled data with a query. So the user input to table is for the smaller table that I want to append.