Recently while working on a custom page we had a requirement to show different screens on the app start based on a choice field in the record. (The custom page was being opened from a button/command on the form)
For the custom page’s App OnStart we were first removing the curly brackets from the recordID parameter passed to set the ContractId variable followed by setting ContractRecord variable using that GUID in the LookUp function.
Below is the JavaScript used to pass the parameters to the custom page (from the command/ribbon button)
Now based on Contract Type optionset field in the Contract record we wanted to show a different screen as the start screen. However, trying to use the Global Variable in the StartScreen function gave us the error – “Global Variables are not allowed in StartScreen”
The solution here was to use the Param recordId to get the record and set the start screen accordingly in the StartScreen instead of ContractRecord global variable.
Also check – https://debajmecrm.com/how-to-dynamically-show-the-start-screen-of-an-app-in-power-apps-canvas-apps/
Hope it helps..
Nishant Rana's Weblog 
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