How to calculate server fingerprint for Filezilla?

When I use Filezilla to connect to the server for the first time, an "Unknown certificate" dialogue appears with the certificate fingerprints. The Fingerprint (SHA-256) value is 32 hex bytes: b3:28:a6:b9:... How do I calculate the server's fingerprint (SHA-256) and have it in the hex format displayed by Filezilla? I tried the following: ssh-keygen -l -f /root/.ssh/authorized_keys The output appears to be in Base64 rather than hex. Here's the first output line (partially redacted): 2048 SHA256:Nm/TDxjNEe_________________________________ no comment (RSA) Thanks.