sometimesonweekends

I have a middle name which I can see on paper correspondence from HMRC eg. tax code notice, but when I log into my personal tax account to use your digital services, I see only my first and last name. Is the online platform separate to what HMRC hold on file elsewhere for me? How do I add my middle name to my personal tax account? I'd like the name to be shown exactly how it appears on my passport, both online and offline.