Access specific appsettings key in Url Rewrite rules - url-rewriting

Question

I am trying to access AppSettings Key in the Url Rewrite rules and I am not sure how to access them. Can anyone help me out?
<appSettings>
<add key="APIUrl" value="https://www.x.com/api/{R:1}" />
</appSettings>
<system.webServer>
<rewrite>
<rules>
<rule name="ProxyApi" stopProcessing="true">
<match url="^api/?(.*)" />
<serverVariables>
<set name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" value="{HTTP_ACCEPT_ENCODING}" />
<set name="HTTP_X_ORIGINAL_HOST" value="{HTTP_HOST}" />
</serverVariables>
<action type="Rewrite" url="{APIUrl}" />
</rule>
</rules>
</rewrite>
</system.webServer>
Trying to access the APIUrl key in the UrlRewrite Rule

Answer 1

I think that appsettings are unavailable elsewhere in your config files.
I've found two ways to address this issue using msbuild:
Use xmlupdate task from MSBuild Community Tasks Project to update config files. My work was using this already so it was the path that I took. Would look Like:
<XmlUpdate
XPath="//rule[#name='ProxyApi']/action/#url"
XmlFileName="{Your Config File Location}"
Value="https://www.x.com/api/{R:1}" />
Use XslTransformation Task to update your config files. This solution is built in but requires more knowledge of XSL. The Xsl would looke something like:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="xml" indent="yes"/>
<!-- identity transform -->
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="//rule[#name='ProxyApi']/action/#url">
<xsl:attribute name="url">
<xsl:value-of select="'https://www.x.com/api/{R:1}'"/>
</xsl:attribute>
</xsl:template>
</xsl:stylesheet>