Please follow the steps below

1. Open the app, click the List Fields link in the left menu
2. In the page appears, click the Add button at the top left
3. In the page appear, you are able to edit/delete the field. Click the Ok button when you complete
4. Click the Request Form Type link in the left menu, click the icon of the approval process that you want to add the field
5. In the Fields tab, click the Add button on the page that appears
6. Select the field & click the Add button to close the popup
7. Click the Update Changes button in the Fields tab
8. If any steps in the Workflows tab that use the feature Can edit fields, edit the step & add the field to the Can edit fields field