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Add a connection string to the S4S system Web service to the <connectionString> element of App_Config/ConnectionStrings.config. It should look as follows 

For Salesforce production and developer environments the connection string should look like:

<connectionStrings>
  <add name="S4SConnString" connectionString="S4S:user id=user;password=password;environment=login.salesforce.com" />
</connectionStrings>

For Salesforce sandbox environments the connection string should look like:

<connectionStrings>
  <add name="S4SConnString" connectionString="S4S:user id=user;password=password;environment=test.salesforce.com" />
</connectionStrings>

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