Azure App Service Reverse Proxy works only on port 80 http

I'm trying to configure Azure App Service Reverse Proxy to expose webserver in Azure Virtual Network to the internet and I had limited success - it's working when I'm not using encrypted connection between reverse proxy and target server.

Here is my currently working configuration:

web.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <system.webServer>
        <httpErrors errorMode="Detailed" />
        <rewrite>
            <rules>
                
                <rule name="ForceSSL" 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>
                
                <rule name="Proxy" stopProcessing="true">
                    <match url="(.*)" />
                    <action type="Rewrite" url="http://example.com/{R:1}" />
                    <serverVariables>
                        <set name="HTTP_X_UNPROXIED_URL" value="http://example.com/{R:1}" />
                        <set name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" value="{HTTP_ACCEPT_ENCODING}" /> 
                        <set name="HTTP_X_ORIGINAL_HOST" value="{HTTP_HOST}" />
                        <set name="HTTP_ACCEPT_ENCODING" value="" />
                    </serverVariables>
                </rule>
            </rules>
            <outboundRules>
                <preConditions>
                    <preCondition name="CheckContentType">
                        <add input="{RESPONSE_CONTENT_TYPE}" pattern="^(text/html|text/plain|text/xml|application/rss\+xml)" />
                    </preCondition>
                </preConditions>
            </outboundRules>
        </rewrite>
    </system.webServer>
</configuration>

applicationHost.xdt

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
    <system.webServer>
        <proxy xdt:Transform="InsertIfMissing" enabled="true" preserveHostHeader="false" reverseRewriteHostInResponseHeaders="false"/>
        <rewrite xdt:Transform="InsertIfMissing">
            <allowedServerVariables xdt:Transform="InsertIfMissing">
                <add name="HTTP_X_ORIGINAL_HOST" xdt:Transform="InsertIfMissing" xdt:Locator="Match(name)"/>
                <add name="HTTP_X_UNPROXIED_URL" xdt:Transform="InsertIfMissing" xdt:Locator="Match(name)"/>
                <add name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" xdt:Transform="InsertIfMissing" xdt:Locator="Match(name)"/>
                <add name="HTTP_ACCEPT_ENCODING" xdt:Transform="InsertIfMissing" xdt:Locator="Match(name)"/>
            </allowedServerVariables>
        </rewrite>
    </system.webServer>
</configuration>

IP Address of the server has been replaced by example.com

When I change web.config rewrite rule and HTTP_X_UNPROXIED_URL to https I get following IIS Error

Detailed Error Information:
Module                         ApplicationRequestRouting
Notification                   ExecuteRequestHandler
Handler                        ApplicationRequestRoutingHandler
Error Code                     0x80072f8f
Requested URL                  https://example.com:80/
Physical Path                  D:\home\site\wwwroot
Logon Method                   Anonymous
Logon User                     Anonymous
Request Tracing Directory      D:\home\LogFiles

This suggests, that for some reason it's trying to request https on port 80.

I tried to follow guide from here: https://www.patrickob.com/2018/11/10/adding-ca-certs-to-the-trusted-root-store-for-web-apps-hosted-in-an-ase/

I have added my organizations root cert to SSL certificated and added WEBSITE_LOAD_ROOT_CERTIFICATES setting.

Any help will be appreciated.

WEBSITE_LOAD_ROOT_CERTIFICATES is only supported in an App Service Environment, its not supported in multi-tenant app services. If the endpoint on-premises is signed by a certificate not using a public CA at this time there's no direct workaround to make this work outside of updating the certificate or possibly writing a simple proxy app that can ignore SSL cert validation (I typically wouldn't recommend this option unless you do the cert validation yourself in code).