If you have already created the field in the table that the form uses
as a record source then just click VIEW-FIELD LIST and a list of all
the fields will pop up in an tiny window. You can then drag and drop
the new field onto your form. Once you type data in the fields and
click off the field the data is saved. Only time that is a problem is
if the user changes data nd then closes the form by the X, in which
case you may want to have a Save Form button on your form.

If its a new field in the table, you'll need to add this field to the
RecordSource of the form, follow this steps:
1. Open the form in design view
2. Go to the form properties, in the menu bar select view -> properties
3. Press on the left top corner of the form to display the form properties
4. Go to the recordsource property of the form, click on the button on the
right that has three dots
5. Now the SQL is open, that include your tables, add the fields you
want to add to the form, unless the RecordSource already select all record
(Select TableName.* From TableName)
6. as you exit the query, it will ask you if you want to save, say yes
7. Before closing the form, save the form, it might close without saving

8. Now, add a text box to the form
9. Select the ControlSource property of the text box
10. Click on the menu on the right and select the new field name, that way
you'll bount the text box to the field in the table