This post is how to get a secret from a key vault in Synapse.
If you have Data Exfiltration enabled (which is recommended) then you need to have a Managed Private Endpoint setup to your KeyVault.
You also need to ensure your Synapse Managed Identity has access to your Key Vault.
You also need a un-parameterized Linked Service Created.
Then you can query your Key Vault to get the secret with the following command.
from notebookutils import mssparkutils
secret = mssparkutils.credentials.getSecret('<KEY_VAULT_NAME>', '<SECRET_KEY>', '<LINKED_SERVICE_KEYVAULT_NAME>')