Customer Portal - Send links to take action without login

I am working to develop an application on Appian where I can send request links for third-party users where they can click on the link, fill a form, provide any supporting documents and give their approvals.

I want to understand how the license structure would work in this case as these third party users would be external users to my organization and I only want to give them access (preferably just for filling the links via link) and not proper user login sessions.

Please help