Am I understanding you want to send mail via Postfix on the local Virtualmin server, rather than using Amazon SES? You’re going to have a really hard time achieving high deliverability on a new virtual machine. You might get blocked immediately if you start sending hundreds or thousands of messages a day from a brand new IP, one with no reputation (or a bad reputation; most VM providers IPs have been burned by spam, you have to rebuild reputation for those). Even if you get everything right with DKIM, SPF, PTR record, etc. You’re probably not going to be happy with results.
A regular mail server that just sends a few mails a day can usually work OK. You spot the rejections during the first couple weeks (probably Microsoft, because they block hard and fast, and with low granularity), request unblocking, never send any spam in the future, and all is well. Bulk mail? Whole other story.
But, if that’s what you want to do, Sendy is just a mail client in this scenario. You give it the same SMTP credentials you’d give a user. I assume you’ve created a mail user for Sendy to send as, in which case you should know the password (you set it, or you can reset it if you don’t know it), and you can see the full username in the Edit Users page of the domain you’ll be using for Sendy. It’s in the column labeled IMAP/POP3/FTP login (will probably be something like sendy@domain.tld).
Port is any of the common SMTP sending ports; since you’re connecting locally (I’m understanding you are running Sendy on the Virtualmin server in a domain, yes?) on IP 127.0.0.1, you can use any of the ports with no risk of being blocked. 25, 465 (SMTP over TLS), 587 (STARTTLS) are all enabled in a default Virtualmin installation, and usually people using mail clients would connect on 587, since port 25 is blocked for so many home and consumer networks. Most folks probably use 25 for local sending, since you don’t even need to authenticate in such a case. It isn’t clear how you’d configure Sendy to just use the local sendmail command from your screenshot, but that’d also work, I’d think. Local users do not need to authenticate to send mail.
One other caveat about hosting your own mail: A lot of VM providers, and all major cloud providers, block port 25. You cannot send mail from a VM running at Google Cloud, Azure, Amazon S3, etc. You have to have a relay (like SES, Mailjet, Mailgun, Sendgrid, etc.).
