How to prevent the Mac from autocorrecting the username

Go into the keyboard settings and add a shortcut for the corrected spelling of the word. I had to do this with my daughter's name, as every time I spelled it, it would come out in capitals. You can just use a part of the word and it can auto spell the word for you, or you can spell it with a different capitalization and save the corrected version the other way.

If you need to use the first part of the word it may not work. Don't really know, but it worked for me ;-)

AutoCorrect will automatically replace the word unless you cancel it manually by tapping it. (Note the small × sign in the suggestion bubble indicating that tapping it will cancel the suggestion.)

So, as soon as an AutoCorrect suggestion appears that you want to use, just hit Space (or insert a punctuation mark) and continue typing.