Pass username & Password through URL

We are a new user having just installed MailWizz today. Our plan is to allow our customers access to the customer portal section via an iFrame within our own secure website that everyone has to login in to.

As customers are already within a secure portal i would prefer not to have to ask them to login again, therefore is there a way to bypass the username and password for just this instance or maybe pass the details through the URL being used in the iFrame.

Ultimately there will also be ( is ) public access through a dedicated URL available directly to the customer section so i can not disable logins ( if that seven possible ) completely.

Any idea's, suggestion or how to do this would be appreciated.

Many Thanks
@Rod Rayner - Mailwizz's forms are protected by csrf tokens to improve security, so if you will post data from other sources, you also have to send the csrf token, which makes things impossible.
The only way, would be to do it via _GET, but that would require some changes in the login form itself.
Do you have any programming experience ?