The Exchange Online microservice requires a key vault with a certificate for certificate-based authentication between the microservice and the service principal registered for it.
Create the key vault and certificate
In Azure, create a Key vault if you do not already have one or want to create a new one.
Navigate to the Certificate page for the key vault and click Generate/Import.
Enter a Certificate Name and Subject and then click Create.
After Azure creates the certificate, click the record for it and then download the certificate in CER format. You will add this to the service principal you created for the Exchange Online app service.
Add an access policy
Navigate to the Access policies blade for the key vault.
Click + Add Access Policy.
Click the Certificate permissions drop-down and select Get.
Under Select principal, click None Selected.
Search for and select the Managed Identity for the Exchange Online App Service. Please note that the Managed Identity you select is the System Assigned Managed Identity you enabled for the app service.
Click Select to select the principal.
On the Add access policy blade, click Add.
Upload the certificate to the service principal
Navigate to the Certificates & Secrets blade for the service principal you created for Exchange Online.
Upload the certificate you downloaded from the key vault.
Next Steps
Provision a Cosmos DB Account for Exchange Online
Add application settings to the app service
Publish the Exchange Online Microservice