I don't know what a "Word form" is. But i'd guess that the answer is that you can't. Thunderbird is not Word, so it can not/will not execute any Word-specific content or scripting.

What features would you put into such a "form"?

My bad Zenos. I have created a form in Microsoft Word 2013 format, which I would like to send through Thunderbird 52.1.1 in the body of the message. I do NOT want to send it as an attachment. The recipient should be able to edit this form (write in it) and reply without attaching it. I tried to add a button to the ribbon in MS Word that sends the document you're working on, but that did not work with TB. I may have to build the form I need inside of Thunderbird, Thanks for your response sir,

You could copy the content of the Word doc and then in thunderbird Write window, use 'Edit' > 'Paste without formatting', but it may look a bit different.

The reason for using attachments is so the recipient can read the document exactly as you intended. Emails sent do not necessarilly look the same on the received computer. The user may prefer a different font or not have the font you have specified, or a different size for ease of reading. They may use Plain Text instead of HTML to read emails. It is worth baring this in mind when sending an email, particulary if you compose forms in a Word Processing Program which offers a load more formatting options. If these 'forms' are something you send out on a regular basis, it may be worth while composing them in thunderbird and saving as a template email, so allowing you to reuse them again and again.