Form() is part of System.Windows.Forms.Form() it works in Rhino 5 + 6 (Windows) but not on Mac.
Dialog() is part of Eto.Forms.Dialog() it works in Rhino 6 (Windows) and on Mac, but not in Rhino 5 (Windows)
the method rs.HtmlBox is a RhinoScript method (vbscript) which works in Rhino 5 + 6 (Win) but is not available within python scripting yet. It also does not work under Mac which uses python.
If you want to get started under Windows, and do not care much about using your dialog on a Mac, start using Windows.Forms. There are many examples online. One simple example dialog together with some tips how to create it without much programming knowledge can be found here.
Btw, ETO is not so different from Windows.Forms as many method names and mechanisms are identical. Once you have gained some knowledge about Windows.Forms, it is much easier to get started with ETO.
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