At Form Edit, please go to 'Customer Details' tab.
Scroll down until you see 'Additional Customer Details' panel.
Click on 'Add New Question' to add a new question. You may add the question and placeholder. The default type of question is 'Short Answer'.
You may change the type of question to: paragraph, checkboxes, options or dropdown.
If you choose Checkboxes, Option or Dropdown, you will need to put some data for selection by your customer.
When you done hit 'Save' button.
You may also set the question as required: so that your customer need to fillup before he/she can proceed.
Other option is 'Show on WhatsApp Message & Customer Order Page'. By checking this option the question and answer will be appeared on WhatsApp message as well as customer order page.
Here is some example of the result: