How to enable the usage of the "From" field within a "Send Notification" action in Nintex Workflow?
1. Navigate to Central Administration > Nintex Workflow Management > Global Settings (2010, 2013), Central Administration > Nintex Administration > Messaging and notifications (2016, 2019)
2. Set the option "Allow sending notifications on behalf of another user" to "Yes"
3. Perform an IISReset
Once the From field has been enabled as per above, the From field will also be displayed in the task notification settings for workflow task actions like Assign flexi task, Request review, etc.