The apps list you have entered in the midSessionInstallApps needs to be an array of the hashed app-id(received as a response) of each app on uploading the app on BrowserStack.
"midSessionInstallApps": ["bs://<hashed app-id>", "bs://<hashed app-id>"]
Note: Maximum 3 app ids allowed. This capability has to be used along with app capability. The app capability value should not conflict with midSessionInstallApps capability.
For more info, you may refer to our capabilities-list documentation