Open up Pamac, your graphical package manager, and click on the 3 lines in the top-right corner.
At the bottom of the window, click Third-Party, then enable AUR support.
Search for Syncterm and it should show up now.
Getting packages not in official repositories is significantly easier in Arch-based distros like Manjaro than Debian-based distros or RPM-based distros. It’s a major reason why I never use either for personal computing.
Disclaimer: I’ve been a Manjaro user for 3+ and seeing people recommend Fedora over Manjaro just further cements in my mind that the vast majority of people in this community are not worth listening to.
Do you notice how many people in this thread are recommending you different distros instead of simple instructions on how to get Syncterm to work with Manjaro?