Noticed I didn’t have an agent-specific one to reference – here it is.

#Assign agents a management server failover list
$primaryMS = Get-ManagementServer | where {$_.Name –eq '<MS1_FQDN>'} 
$failoverMS = Get-ManagementServer | where {$_.Name –eq '<MS2_FQDN>'} 
$agent = Get-Agent | where {<add some criteria here>} 
Set-ManagementServer -AgentManagedComputer: $agent -PrimaryManagementServer: $primaryMS -FailoverServer: $failoverMS