Command needed to Force Group Policy to Process on all domain computers

gpupdate /force

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copy the line and save it into a "name.bat"

and you can save the file into a new GPO and add it on the login script or logoff script
https://www.websense.com/content/support/library/web/v78/logon_agent/la_configure_scripts.aspx

I need a command that will update all computers in a domain not just one computer

Using GPO will update all computers
That's the reason why you do it as a login or logoff script

https://docs.microsoft.com/en-us/powershell/module/grouppolicy/invoke-gpupdate?view=win10-ps

#Run from a domain controller

Import-Module ActiveDirectory
Import-Module GroupPolicy

Get-AdComputer -filter {enabled -eq $true} | select DNSHostName,Enabled | %{
    if(Test-Connection -Count 1 -ComputerName $_.DNSHostName ){
        Invoke-GPUpdate -computer $_.DNSHostName 
    }
    else{
        Write-Host -ForegroundColor Red "Computer $($_.DNSHostname) is offline"
    }

}

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If I run this as a log in script in a GPO, doesn't the user need local administrator credentials vs a computer start up script does NOT require any credentials for it to run ???

It depends upon GPO setting configured
Not all computer settings get updated with gpupdate /force because some settings only update during computer reboot / startup
Either run above script from domain controller manually or wait for computer reboot, upon reboot computer must refresh all computer based GPO

You COULD construct a GPO to edit the GPO refresh interval. Of course, it needs to fire in the usual time before the new refresh rate applies.

The default is 90 mins, with a random 30 min offset. The offset stops all machines for trying to process GPOs at the exact same time in the event of returning from a power failure.

Although Windows will let you drop the refresh period to as low as 7seconds, this tends to annoy users, and can generate a lot of traffic if you have many machines and a slow network link. The user desktop redraws where GPOs are applied, users usually don't notice.  When trying out new GPOs, I have set this to 5mins in the past. That is a reasonable compromise. Change a setting, wait 5 mins, all machines will have applied it.  Of course, you can change this back once you finish playing.

More here: https://social.technet.microsoft.com/wiki/contents/articles/31623.modify-group-policy-refresh-interval-for-computers.aspx

you can do with group policy management console or power shell

Power Shell cmd
Get-ADComputer –filter * -Searchbase "ou=LAB, dc=lab,dc=local" | foreach{ Invoke-GPUpdate –computer $_.name -force}
gpo.docx

@Lee:
Thanks for reminding option

Thank you all for your efforts. I agree it's  a solution. I have to test it in terms of network usage.