Need help to understand express route provisioning.

We've successfully configured the Express Route in Azure and obtained the service key (S-key) for connectivity. However, I'm seeking clarification on whom I should provide the S-key to for further configuration.

Should the S-key be provided to the internet provider responsible for the on-site connection, or should it be handed over to the individual managing the on-site Meraki MX75 firewall? Additionally, I'd like guidance on the specific configurations required on the on-site Meraki firewall to ensure seamless integration with the Express Route.