One way to go about doing this kind of thing is to add some settings to the plugin they are installing such as the following:
- (Blankman’s Site – Your User Name)
- (Blankman’s Site – Your Password)
- (Blankman’s Site – User Name to access your site)
- (Blankman’s Site – Password to access your site)
The user could then add in the first two values and, if set, they could be used to automatically log them in when they click the link to your site before actually directing them there, most likely via javascript, curl, etc.
Then, if they set the user name and password for your site to access their site, you could encode / encrypt it and transfer it to your site during their log in, unencode / decrypt it, and store the credentials somewhere for use in accessing their site.
Of course, they would have to create a user account on your site and give you a set of permissions.
This would a clean, up front method of doing it.