if by software update you mean macOS, they all need a reboot.. swift dialog / nudge are UI options.. or use JAMF beta software update via DDM.  Not sure your reference to 'Start' and 'Complete' fields.. 

I am sorry if I was unclear, let me rephrase.

I am creating a policy that will apply updates to macOS.  I want it to check for updates, and apply them silently, and only send a pop-up to the user if a reboot is required.  

When creating the policy, there is a field called "User Interaction" at the top. The menu looks like this:  Options / Scope / Self-Service / User Interaction
Under User Interaction there are fields Start Message, Complete Message, and Restart Message.  I want a message to be displayed only if a restart is required.  So, I am asking, if I can leave the first two fields blank.

ignore all of that.. its deprecated to the point it does not work.. unless your on a very old and insecure OS. macOS updates can be actioned via DDM / JAMF Software update, or by using Nudge or with code and SwiftDialog (which Nudge uses).

the JAMF policy for software update is dead.. 

The only way (currently) to trigger software update with a policy is from CLI, which will not work on Apple Silicon Macs. The command for Intel Macs would be "sudo softwareupdate -aiR", the -R is the reboot command. The only way to install software updates is to trigger the reboot with the install command, which prevents you from passing the reboot to the user. A reboot trigger passed from any location other then the software update task sequence will fail OS updates and they wont install. 

The only way for the user to control the reboot is to use a MDM Command or DDM, and they can control the reboot with the macOS notification. 

Note: The "sudo softwareupdate -aiR" command tehcnially works in Apple Silicon Macs, but it prompts a credentails check to the user for a Secure Token. If the user does not have a Secure Token or cancles the popup the OS updates fail.

Hey @Coyote2045  Here's another user friendly way to automate user interactions for JAMF policies/actions via Slack/Teams: https://www.amplifiersecurity.com/for-jamf

This is an AI engagement & automation layer for JAMF to interface with the workforce. You can design the tone/language/custom messages as AI prompts and let Ampy (an AI security buddy for employees) do all the heavy lifting to engage with your end users related to overdue JAMF actions. You can let users schedule such JAMF actions via Slack/Teams and boost user productivity by giving them more control on when JAMF policies can run!