Target and replace XML connectionStrings node

[RaviRamDhali]

I am attempting to change a cnfgConnection.config > "connectionString" value using the following action and settings.

How do I target the connectionStrings > add > connectionString value?

- uses: microsoft/variable-substitution@v1 
   with:
       files: './Website/cnfgConnection.config'
   env:
          connectionStrings.add: "Data Source=prod.server.com;Initial Catalog=test;User ID=prod_tester;Password=xxxxxx"

Example XML

<?xml version="1.0"?>
<connectionStrings>
  <add name="DefaultConnectionString" connectionString="Data Source=test.server.com;Initial Catalog=test;User ID=tester;Password=xxxxxx" providerName="System.Data.SqlClient"/>
</connectionStrings>

[RaviRamDhali]

I got it working with the following code:

- uses: microsoft/variable-substitution@v1 
   with:
       files: './Website/cnfgConnection.config'
   env:
          connectionString: "Data Source=prod.server.com;Initial Catalog=test;User ID=prod_tester;Password=xxxxxx"