You must set this configuration to specify which SMART on FHIR application configuration the Application Server should use when performing SMART on FHIR authentication. This setting can be found in the Application Server's web.config file, in the <configuration>|<appsettings> section.
<configuration>
<appSettings>
<add key="SMARTonFHIRApplicationId" value="[GUID]" />
</configuration>
The value for the GUID can be found in the Application Unique ID field in the Application Server Settings. See step 3 in Creating a SMART on FHIR Application to locate the application ID.