Seems you will have to update Apache setup and include the subdomain there.
Installing Sendy is little tricky. For us it took around 2 months to properly understand, but, fortunately, we came across a service called EasySendy, which automated everything for us.
Add my skype ocbineesh I can fix your issue.
I've documented this exact case in a complete guide to installing Sendy. I've included a lot of detail that helps you understand what you are doing, and why. Please check it out here:
It also covers how to make automated backups of your database and automate sending those to an AWS S3 bucket, as well as using certbot to generate SSL certs for https.