In my osticket I can't see the SMTP configuration options.
Is it necessary to install any addon?
You are not on the page for setting up SMTP.
Click on Emails.
Click on the Email you want to configure.
Once you are done doing that go back to the screen you posted and set your Default MTA
Done, thank you.
So you resolved your issue?
I've marked this thread as resolved and closed it. Please feel free to start a new thread if you have another question.