I spent quite a while looking for a DSC (Desired State Configuration) module to set the Primary DNS Suffix without much luck.
Specifically, I was looking to set this field:

This is what I managed to get working.
$dnsSuffix = "contoso.com"
Script SetDNSSuffix  
{
    SetScript = 
    {
        Set-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\" -Name Domain -Value $dnsSuffix
        Set-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\" -Name "NV Domain" -Value $dnsSuffix
    }   
    TestScript = 
    {
        $currentSuffix = (Get-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\" -Name "NV Domain" -ErrorAction SilentlyContinue)."NV Domain"
        if ($currentSuffix -ne $dnsSuffix){
            return $false
        }
        return $true
    }   
    GetScript = 
    {
        $currentSuffix = (Get-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\" -Name "NV Domain" -ErrorAction SilentlyContinue)."NV Domain"
        return $currentSuffix
    }     
}
- SetScript - Fires when the value needs to be set
- TestScript - Fires to test whether the value needs to be set
- GetScript - Gets the currently set value