Where is Add-MgServicePrincipalKey source code?

[qizu0302]

Currently when I run "Add-MgServicePrincipalKey -ServicePrincipalId $spoappid -BodyParameter $params -Verbose", it always shows error “Add-MgServicePrincipalKey : Resource '00000003-0000-0ff1-ce00-000000000000' does not exist or one of its queried reference-property objects are not present.”

In my command, $spoappid is '00000003-0000-0ff1-ce00-000000000000', while $params is
$params = @{
keyCredential = @{
type = "AsymmetricX509Cert"
usage = "Verify"
key = [System.Text.Encoding]::ASCII.GetBytes("MIIDYDCCAki...")
}
passwordCredential = $null
proof = "eyJ0eXAiOiJ..."
}

I wonder where I can find the source code of Add-MgServicePrincipalKey so that I can troubleshoot my error?