The documentation for Fedora 40 CoreOS can be found on the official webpage.
I download the fedora-coreos-40.20240331.1.0-virtualbox.x86_64.ova file.
You can use double click to open with the VirtualBox application, see the next screenshot:
Fedora use ssh keys and a user named core, use these steps from the official webpage to change the password for this user.
The result will be this:
You need to use these steps to create a password based on the butane and ignition software to login with the ssh-rsa.