We have no way of knowing what page you are looking at. What is "the form you have shown me" that you referred to?
When you fill out your tax return and go the My Info tab, there is indeed a data field to enter a middle initial, so you'll be able to insert it there.
You referred to "the signin." The actual log-in screen doesn't require an actual name at all. The User ID can be whatever you choose for it to be.
Or are you referring to the Account Settings PROFILE section? If so, then that screen does only ask for first and last names, but you can still include a middle initial when you fill out your return.
