Winform solution and access to app.config

I have a startup project which is a Windows Application and a class library project which handles data access.

I have placed a connection string property in the Windows Application app.config file.

<configuration>
  <connectionStrings>
    <add name="ConnectionString" connectionString="Data Source=POWDB..."  providerName="System.Data.SqlClient" />
  </connectionStrings>
</configuration>

I need to access the connection string from the class library. I thought I could write something like:

_connString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString; or 
_connString = ConfigurationManager.ConnectionStrings["PowRev.Main.Properties.Settings.ConnectionString"].ConnectionString;

But at runtime it chokes, putting out a messagebox saying it can't find the source to display. After hitting the OK button, the program exits.

What is the proper way to do this? 

Thanks, Lars

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