Here is an example of a shortcode with the square brackets "[" and the "]" stripped out.
i4w_m_autologin site='' img='' redir=''
In my case I'd do something like...
i4w_m_autologin site='http://gateway.responseflow.com' img='http://infusionsoftmastery.net/resources/img/membership/sidebar9_gateway.jpg'
site: the site you'd like the person logged into ie: http://mysite.com or http://mysite.com/my_wp_install_directory
img: the full URL of the image you'd like people to click on to log into the other site ie: http://mysite.com/images/button.jpg
redir: Optional, used to specify what page a person will go to after they become logged in ie: http://mysite.com/other-page/