I have a tasks table that I want to send a notification when the assigned user is changed, to let them know that the user has a new task
but the assigned to user field is not in the specific fields change option list
So, the [Assigned To] is a formula field? Can you post the formula of the [Assigned To] field here? Also - it looks like you might be using [Related Resource] fields to indicate an Assigned To from another table. If this is true, your trigger would be when the [Related] reference field has changed, not the formula-output field.