Hello there,
if you wirte “</textarea>” into the text area if you create a site and save it, it will crash your site.
Somehow it will be used by kirby itself in the panel (it closes the textarea-tag from the panel) and wont show up anymore in the text field.
Therefore you can’t use that tag…
Well, that may be although it in my install it doesn’t crash but the the closing tag is not saved to file.
However, I wonder what your are trying to achieve? It would probably be better anyway to include a snippet with a form rather than individual fields that don’t have any matching code to handle the input?
I’m new to Kirby and played around with a site. Testing a registration form.
But it doesn’t seem like a good way including HTML in you site via Kirby’s panel.
I just wanted to report that bug. 
In general, you can use HTML in a Markdown document without any problems. While the textarea thingy is probably a bug, IMHO it just doesn’t make sense to include form elements within a text file.
What I would do if I wanted to include a form within a textarea field: create a snippet and use a custom Kirbytag (http://getkirby.com/docs/advanced/kirbytext) to include the form into the text file. That way, you can make sure that there is a script that handles form submission.