Hi I'm trying to better understand which SSO authentication method I should use and would really appreciate the help from someone more knowledgeable than me. 

I am trying to have the customer portal be accessed with credentials (login/password) that they have on my existing wordpress website. BUT if they don't have a username/password on our website, I want them to be able to use the "Sign Up" Feature in the Zoho Desk customer Portal. I already have Zoho Desk on my subdomain.

Which Authentication method needs to be set-up for this? Any help would be very much appreciated.