Blimey. You are COMPLETELY confused, aren't you.
If postfix (the package named "postfix") is installed, and if sendmail
(the package named "sendmail") is NOT installed, then you are using
Postfix to send mail.
Part of the postfix package is a /usr/sbin/sendmail program which
implements the command line interface for local programs to send mail.
EVERY MTA has to implement the /usr/sbin/sendmail program.
Including Postfix.
If you're running Postfix (*not* Sendmail) as your MTA, and if you've
got it configured how you want it, then you are DONE. You don't need
to ask us how to configure Sendmail to do the same thing, because you're
not USING Sendmail.