Maybe something was changed, but until now, you just need to check the box in project details tab, when you are uploading files, where is written "If you have a right model for the challenge, simply check the box and you are in.", and be sure that a tag "homeprinting" has been added.
And if you have more than 10 models uploaded, as it says in the rules, they will also include you in the portfolio part of the competition.