We’re fixing the issue with missing $EventData in 1.4 but until then you can use the $Body variable.
New-UDCheckBox -id "Application_servers_$($Server)" -Icon $Icon -CheckedIcon $CheckedIcon -Style @{color = '#2196f3' } -Label "$($Server)" -LabelPlacement end -OnChange {
Show-UDToast -Message $Body-Duration 600
}