All Rebex components support .NET Standard 2.0, which is supported by all Xamarin platforms, so there is no need to use any kind of dependency injection.
Just reference Rebex assemblies in "netstandard2.0" from your Xamarin application, or add a reference to Rebex.Ftp on NuGet.org, and it should work fine.
Note: On Xamarin.Android, it might be useful to reference Rebex.Common.Native and enable native extensions.