What's the default user and password for administrator?

[bonart]

I don't know the username and password to access the admin area.

[SKaero]

Roundcube doesn't come with a admin area and there is no default user or password. If your referring to some other control panel like cPanel you'll need to contact your email provider to recover access.

[bonart]

But there is like an admin user where you can add domains to use roundcube and some other configuration stuff I had it but the problem is that I installed it a while ago but now I need to add some other domain but I forgot the username and URL to enter into it.

[SKaero]

That isn't part of Roundcube, you'll need to to contact you email or hosting provider to get that information.