URL Rewrite 500.52: Unrecognized element 'serverVariables' - iis-7

Question

I need to implement a reverse proxy using only my web.config and I'm having trouble with the 500.52 error. As suggested in many places online, I added the serverVariables section to include the HTTP_ACCEPT_ENCODING variable, but I am met with an error:
Config Error Unrecognized element 'serverVariables'
Below is my web.config.
<rewrite>
<allowedServerVariables>
<add name="HTTP_ACCEPT_ENCODING" />
</allowedServerVariables>
<outboundRules>
<rule name="reverseProxy">
<match pattern="http://linkInCode.com"></match>
<serverVariables>
<set name="HTTP_ACCEPT_ENCODING" value="" />
</serverVariables>
<action type="Rewrite" value="https://linkIWant.com/script.js"></action>
</rule>
</outboundRules>
</rewrite>
What is the problem? ServerVariables should be a recognised element.

Answer 1

You need to have the server variable registered previously in <allowedServerVariables>,
https://learn.microsoft.com/en-us/iis/extensions/url-rewrite-module/url-rewrite-module-20-configuration-reference#Allowed_Server_Variables_List

Answer 2

Using this blog post, I copied the examples given and changed sections for my needs.
<rewrite>
<rules>
<rule name="Rewrite to article.aspx">
<match url="^article/([0-9]+)/([_0-9a-z-]+)" />
<action type="Rewrite" url="https://URLtoHide.com" />
</rule>
</rules>
<outboundRules>
<rule name="Rewrite to clean URL" preCondition="IsHTML">
<match filterByTags="A" pattern="http://http://URLtoShow.com" />
<action type="Rewrite" value="https://URLtoHide.com" />
</rule>
<preConditions>
<preCondition name="IsHTML">
<add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
</preCondition>
</preConditions>
</outboundRules>