What you are really asking is whether you can create a web page that is included on your sitemap that has a form and when clicking on “send” it posts the contents of the form to an Item, right?
I can’t help with the form part but I can say that it’s possible. If you look at OAuth2 using just OH Rules and myopenhab.org, you will see that I have a call to set an OH Item when a web page get’s loaded. In this case the page get’s loaded through myopenhab.org with an argument, but you should be able to use it as an example to build up something that might work for you.