@Milind , thanks for reaching out to us. Please note that Custom Domain
is supported only on Basic or higher-tier plans. F1:Free tier doesn't support the custom domain. If your app service plan doesn't support the custom domain, you'd get an error as shown in the image below:
To choose the right Service plan, you can follow Choosing a Right App Service Plan. Further, to add a sub-domain, you can follow this step-by-step tutorial: Adding Subdomain to App Service.
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