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After Steps 1 and 2, you will see a list of apps that are registered in your sandbox. Select the app you would like to create the launch scenario for. You can use the Custom App to launch an app that has not been registered. You can use the client my_web_app for development prior to registering your app and gaining your own client id.
Step 4: Save or launch
If you choose to save your launch scenario, it will be available for repeated execution. If you don't want to save it, you can simply launch without saving.