My Own login Form

I am hosting my simple html web page files on an account sold by a re-seller hosting package. I would like my my email users to go to the website and go to the login page fill in username and password information click on the login button so that it takes them direct to roundcube webmail inbox. After they logout they are re-directed to my website rather than cPanel logout page. I would like to avoid cPanel Login and Logout Pages. How can I do that?