Name in Personal Tax Account on HMRC Online

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.

Hi,

The online platform only shows first name and last name.

Thank you.