When the document must be signed by an external user, you will simply have to indicate their email address when writing the application in the Recipients section. Viafirma Inbox will send you an email where you will be invited to access a signature page where you will be able to review the document and choose one of the signature mechanisms among those allowed for each case.
They will not be able to access the system but will be able to sign applications via a link that will be sent directly to their e-mail address.