Can we not set Allowed Headers for Azure App Service?

Azure App Service allows to configure CORS for web application, but it doesn’t provide a direct option to set Allowed Headers in the Azure Portal UI.

To handle CORS in backend, we can set the allowed headers programmatically based on the framework.

Below is the Example of Asp.net core, I set the Cors in Program.cs File.

Program.cs:

builder.Services.AddCors(options =>
{
    options.AddPolicy("CustomCorsPolicy", policy =>
    {
        policy.WithOrigins("https://FrontendAppURL")
              .AllowAnyMethod()                        
              .AllowAnyHeader()                       
              .WithHeaders("X-Custom-Header", "Content-Type");  
    });
});

app.UseCors("AllowCustomHeaders");

For .Net Framework I used web.config file to set the Cors in Azure Web app service.

I added the web.config file to the root directory of the Web Api.

web.config:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <clear />
       <add name="Access-Control-Allow-Origin" value="*" />
        <add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE, OPTIONS" />
        <add name="Access-Control-Allow-Headers" value="Content-Type, X-Custom-Header" />
        <add name="Cache-Control" value="public, max-age=3600" />
        <add name="X-Custom-Header" value="MyCustomHeaderValue" />
      </customHeaders>
    </httpProtocol>
    <rewrite>
      <rules>
        <rule name="Redirect to HTTPS" stopProcessing="true">
          <match url="(.*)" />
          <conditions>
            <add input="{HTTPS}" pattern="off" ignoreCase="true" />
          </conditions>
          <action type="Redirect" url="https://{HTTP_HOST}/{R:1}" redirectType="Permanent" />
        </rule>
      </rules>
    </rewrite>
    <defaultDocument>
      <files>
        <clear />
        <add value="index.html" />
      </files>
    </defaultDocument>
  </system.webServer>
</configuration>

Output:

For More Details refer this Ms Doc.