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.
Keep going thru the questions
.....Actually, when starting from scratch...no data transfer, the software first asks only your first and last names (or was it just the First name....I went thru that a day or two ago...)...then later, after some minimal starting information, you'll be able to enter everything and the middle initial....
But if you do miss it somehow, then as [mention://5807259 @epo39b] indicated, you can just go to the (edited) "My Info" section and enter it there.
embrandt:
I edited my original comment above. The "Personal Info" section for 2017 is now called the "My Info" tab. When you open your return interview and are past the blue "Simple and accurate" screen, you'll see it in the left column menu.