The built-in onOpen trigger in Google Apps Script only runs when a user with permission to edit opens a spreadsheet or document. I am developing a spreadsheet where only developers have edit privileges and all other users must have view-only privileges.
How do I create an onOpen trigger that fires when view-only users open my spreadsheet as well as those with edit privileges?
You can't, because it's not supported. Google Apps Script, embedded in either Google Sheets, Docs, Forms, and as an add-on, runs only for editors.
The alternative is to Publish as a webapp, so the viewers open that instead of the sheet directly, or open the webapp (on another tab, not inside the sheet) from a link in a sheet cell.
From the "Restrictions" section of trigger documentation on triggers - both simple and installable:
They do not run if a file is opened in read-only (view or comment) mode.**
