You simply run the new account wizard. https://support.mozilla.org/en-US/kb/automatic-account-configuration

If the wizard cannot detect the correct server settings, choose 'Manual Configuration'. https://support.mozilla.org/en-US/kb/manual-account-configuration

Hello christ1, and thank you for your answer. My problem is, that despite i put the user name, the email address, and the password, thunderbird after the checking, appears a message telling me that the user name or the password is incorrect. I put the same user name as the first account. I have checked with different user names, but it comes to the same result. I have checked also the password (the password i have from my email provider). Same result. I also make a manual configure, and put the exactly same income, outcome server names as the first account. Nothing.... I cant understand what i am doing wrong...

I put the same user name as the first account.

If you want to add a new account, then the user ID can't be the same as for an existing account. I don't think I understand exactly what you're doing, so can you be a little more specific please? What email provider are you using? Can you post your Troubleshooting Information? Help (Alt-H) - Troubleshooting Information

Apparently, the problem was from my email provider. The internet connection i have, provides me also with five different accounts. I had to login to their website, and activate a second account, with its user name and password. After that, i put the new data to thunderbird, and the second account was created immediately. Thank you for your replies christ1.

Good. Can you then mark this thread as 'Solved' please? Thank you.

Yes.., thank you  :)