Thanks for asking question!
- If you have purchased an App Service certificate which is private certificate that's managed by Azure.
It combines the simplicity of automated certificate management and the flexibility of renewal and export options.
Refer to these step by step document: Import an App Service Certificate - If you already have a private certificate from a third-party provider, you can upload by following this document:
Upload a private certificate Also, See Private certificate requirements.
Let us know if further query or issue remains.