You need to create a web.config file in your wwwroot folder and put the relevant config entries there.
Here's an example of an web.config rule, to give you an idea of what it should look like.
The below example redirect the default *.azurewebsites.net domain to a custom domain (via http://zainrizvi.io/2016/04/07/block-default-azure-websites-domain)
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="Redirect rquests to default azure websites domain" stopProcessing="true">
<match url="(.*)" />
<conditions logicalGrouping="MatchAny">
<add input="{HTTP_HOST}" pattern="^yoursite\.azurewebsites\.net$" />
</conditions>
<action type="Redirect" url="http://www.yoursite.com/{R:0}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>
Hope this helps!!
If you need to know more about Azure, then you should join Microsoft Azure certification course today.
Thank you!!