I think the general workflow would be to create the task in your account and then use the Tasks service to Share that task with the supplied useremail. I do this all the time when generating Google documents via script. The final stage of the generation shares the document, gives the user ownership and then removes myself as an editor of the document. It's convoluted, but is really the only way (currently) to use all Workspace functionality with Appsheet as it doesn't support service accounts.
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