SenhorDolas-2197, To better understand your scenario/requirement, when you say, “How to create an Azure App Service Certificate (ASC) from CSR?” and …” which I needs the SSL cert renewed.”, Have you purchased ASC already (since you’re referring to renewing certificate)?
1/2
You cannot create an Azure App Service Certificate. It’s a private certificate that’s managed by Azure.
However, you can buy and export ASC to be used on your web server. ( I believe this is what you’re looking for).
Export App Service certificate
Because an ASC is a Key Vault secret, you can export a copy as a PFX file, which you can use for other Azure services or outside of Azure (as in your case).
However, the exported certificate is an unmanaged artifact. App Service doesn't sync such artifacts when the ASC is renewed.
You must export and install the renewed certificate where necessary.
By default, App Service certificates have a one-year validity period. Before and nearer to the expiration date, you can automatically or manually renew App Service certificates in one-year increments.