How do I get the thumbprint of an SSH2 public key on windows?

I have a SSH2 public key with a .PUB extension and when viewed in notepad it has this header:

 ---- BEGIN SSH2 PUBLIC KEY ----

How do I view the thumbprint of this key?

I've attempted ssh-keygen -lf key.pub but that returns: "Is not a valid key file". (I got the binaries for ssh-keygen from github's standard shell.

I'm a bit new to SSH/SSH2 and not sure what the difference is, or if I'm using the wrong version of a tool for the wrong version of a public key.