Did you uplooad the certificate to the keyvault?
Invoke-AddCertToKeyVault -SubscriptionId <guid> -ResourceGroupName westus-mykeyvault -Location "West US" -VaultName mywestusvault -CertificateName mycert -Password "<password>" -UseExistingCertificate -ExistingPfxFilePath "C:\path\to\mycertkey.pfx"
After that - you'll be able to fetch the thumbprint as specified in the guide.
Name : CertificateThumbprint
Value : E21DBC64B183B5BF355C34C46E03409FEEAEF58D
Name : SourceVault
Value : /subscriptions/<guid>/resourceGroups/westus-mykeyvault/providers/Microsoft.KeyVault/vaults/mywestusvault
Make sure to follow all the steps in the guide you listed and you should have your inputs.