[A] In order to do it, go to Settings module >> click on Document Fields under Advanced Settings.
[B] Select the document from the drop-down in which you want to add the field and click on the "Add Field" button.
[C] After that a pop-up will show up, fill all the information about the custom field in it.
like you want this custom field for domestic documents or foreign documents.
[D] Add the name of the field, if you want to add the description you can add it
[E] Now select the data type for that custom field
[F] Select the Mandatory option from the bottom if you want to make that field mandatory in the document and click on "Save Field"