Hi @Thamer Doulat ,
To have your app service call another app service that's hosting your REST API, you need to add CORS functionality. You can use the Azure CLI command az webapp cors add --resource-group myResourceGroup --name <app-name> --allowed-origins 'https://<your-backend-rest-api>.azurewebsites.net'
You can also have Allowed Origins set to '*' to enable any host.