This actually should work. You just need to copy the content of the pem file to the private part of the key when copying it into Termius app in the Keychain section.
We’ll double-check it.
If you have any other problems please contact us here – http://support.termius.com/
It prompted me for a password because I entered, within the Termius host configuration, the user as ec2-user@<fqdn>. Termius also appends the fqdn to the user name so it ended up getting appended twice. It shows you the username it used in the pop up when it asks for the password.