Hello @Salves ,
You can first create an Azure ExpressRoute circuit following this article and once the circuit is created in your Azure subscription, you can check the Overview page of your circuit, where the service key appears in the service key field. You must copy the service key for your circuit and pass it down to the service provider to complete the provisioning process.
Once the ExpressRoute circuit is fully provisioned by the connectivity provider, you can create the peerings. If your connectivity provider offers managed Layer 3 services, you can ask your connectivity provider to enable Azure private/Microsoft peering for you. However, if your connectivity provider does not manage routing for you, after creating your circuit, you have to proceed with the next steps as described in this article.
Please refer the below articles for more information on Azure ExpressRoute prerequisites and workflows:
https://video2.skills-academy.com/en-us/azure/expressroute/expressroute-prerequisites
https://video2.skills-academy.com/en-us/azure/expressroute/expressroute-workflows
Kindly let us know if the above helps or you need further assistance on this issue.
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