trying to get value from secret file .NET 4.8 (VB)

I have a legacy web application in .NET 4.8 (VB). I need to add secret file and read value from it.

In web.config and I added the following:

<configuration>
  <configSections>       
  <section name="configBuilders" 
             type="System.Configuration.ConfigurationBuildersSection, 
             System.Configuration, Version=4.0.0.0, Culture=neutral, 
             PublicKeyToken=b03f5f7f11d50a3a"
             restartOnExternalChanges="false" requirePermission="false" />
  </configSections>
    <configBuilders>
        <builders>
            <add name="Secrets" userSecretsId="0c09afe4-6f59-4293-962b-2aad41d88772" type="Microsoft.Configuration.ConfigurationBuilders.UserSecretsConfigBuilder, Microsoft.Configuration.ConfigurationBuilders.UserSecrets, Version=1.0.0.0, Culture=neutral" />
        </builders>
    </configBuilders>

And then created secret.xml file:

 <?xml version="1.0" encoding="utf-8"?>
    <root>
      <secrets ver="1.0">
         <secret name="Value1" value="someexampletext" />
     </secrets>
    </root>

Then I'm trying to get this value:

Dim s2 As String = ConfigurationManager.AppSettings("Value1")

But get Nothing (the same as null).

I think because I added it to configuration in web.config it I can read it using ConfigurationManager, correct?

Did I miss something?

Adding key to appSettings in web.config resolves problem:

<appSettings configBuilders="Secrets">
    <add key="Value1" value=""/>
 </appSettings>