If you write it in a text-editor such as Word or Google Docs, I recommend reading over the entire thing carefully after you transfer it to ensure there weren't any errors when copy/pasting it into the portal. Some of my punctuation marks were changed to different punctuation marks and a lot of the question marks were turned upside down somehow. Beware! Also, read all instructions carefully as Norfolk63 suggested.
At BGO school, Admissions IT folks advise composing the essay in Notepad and then cutting and pasting into the online application. 2024Candidate is correct about unexpected characters popping into the document.