You will need to access your webmail by going to your-domain.com/webmail.
Log into your email account a username (which is the full email address) and password.
Click "Enable AutoLoad" under the one of the three webmail clients that you prefer.
Choose the seconds that you like for the delay before your preferred webmail client, if you would like it to automatically load your client after logging into your webmail, then choose "1".
To Disable AutoLoad, simply follow the directions above and click "Disable AutoLoad".