In a Confirmation/Notification Email, you can submit data into the Subject line. To do this, first, create a custom Confirmation or Notification Email inside the Settings > Emails & Actions tab of your Form.
In the Form Fields Panel, use the search bar or scroll down to find the field that you would like to add to the Subject.
Drag and drop the field into the Subject line. You can add as many fields as needed as well as additional text. To remove a field, click on the X by the field label. The Subject line will now populate with the data that is inserted into the form.