In case the multhoster service is supported by JDownloader, that means you can just add your account for it in Settings->Accounts, then just add normal links and don't do any link converting on your own.
In Short: if supported, just use JDownloader normally
if not supported, you either have to use the website of the service to convert the link or send us username/password/details to
support@jdownloader.org and we will check/add support for it in JDownloader