Hello ASR,
Thank you for contacting Microsoft Q&A Community.
From my understanding, you would like to know how to assign a role to your App service on Azure Key vault.
To be able to do this, you need to create a managed identity on the Web App and assign a role to the App Service on your Key vault.
Follow this link to see the steps on how to create a managed identity on App Service
https://learn.microsoft.com/en-us/azure/app-service/overview-managed-identity?tabs=portal%2Chttp
Once that is done, follow the steps in the link below to assign a role to the App on the Key vault.
https://learn.microsoft.com/en-us/azure/key-vault/general/rbac-guide?tabs=azure-cli
Let me know if further assistance is required.
Babafemi