Login to the cpanel>> navigate to the email option>> search “Email accounts” option

Under the email section>> locate the email account for which you want to configure the email settings >> click on “check mail” option.

A pop up menu will appear of “open my inbox”>> Mark the check box for the ”open my inbox when I log in option” for Roundcube

Logout and try relogin to the email account freshly and after relogin you will be directly redirected to the roundcube email account.