If you want to add a custom provider to the unstable testing version, store a JSON file like the following under %PROGRAMDATA%\Marcs Updater\ServiceProviders\dyndns.json:

{
    "name": "DynDNS.org (Test)",
    "endpoint": "https://members.dyndns.org/nic/update",
    "method": "GET",
    "queryString": "hostname={HOSTNAME}&myip={MYIP}",
    "dataPayload": "",
    "defaults": {
        "username": "test",
        "password": "test",
        "hostname": "test1.customtest.dyndns.org"
    }
}

Supported query and payload tags are {USERNAME}, {PASSWORD}, {HOSTNAME} and {MYIP}. Restart the background service and foreground application to apply any changes.