You need the contents of your .pub file. Pick the command for your OS and paste the output above:

macOS / Linux
  cat ~/.ssh/id_ed25519.pub | pbcopy   # macOS, copies to clipboard
  cat ~/.ssh/id_ed25519.pub            # Linux, then select + copy

Don't have one yet?
  ssh-keygen -t ed25519 -C "you@laptop"
  # press enter on every prompt for the default location

Windows (PowerShell)
  Get-Content $env:USERPROFILE\.ssh\id_ed25519.pub | Set-Clipboard

Paste the output of the command, not the command itself. The line should start with ssh-ed25519 or ssh-rsa and look like ssh-ed25519 AAAAC3Nza... followed by an optional comment.