Navigate to Customization > Lists, Records and Fields > Transaction Body Fields > New to create the custom field. Then fill the necessary fields such as label and Id. And select the checkbox ‘sale’ under the ‘Applies to’ subtab and also fill the display settings. Afte filling the necessary details select ‘save and apply to forms’. It will redirect to a page containing a lot of forms of sales order. From that page we can select the RMA records. Tick the ‘show’ checkbox near the RMA records and save. Then the created custom field will be displayed in the RMA records.