It is possible to login to webmail from cPanel directly. This is important for scenarios where changing the webmail password may be too cumbersome since it may be configured for IMAP in so many devices.
To automatically login (i.e without password) , follow the guide below:
1.Login to cPanel
2.Go to Email Accounts
3.Click Check Email next to your email address
4. Click Open button next to RoundCube as shown below
Your should be auto logged in
If you need to automatically login next time as soon as you click Check Email button, select the option Open my Inbox when I login
Video #
Below is a video of the process