You can't. You can only flag with specific reasons provided.
Custom reasons are only available for people with cast close and reopen votes privilege.
It is a design decision to prevent too much bad creativity from the people with too low reputation. Reputation measures (roughly) how much community trusts you, and it takes quite some (3000) trust to allow closing because you feel it's off-topic. Sometime question is about issue in existing module, but OP actively tries to fix it and simply stuck at some point. Or is looking for alternative modules to achieve the same goal. These can be pretty OK - it's a delicate matter that requires trust (reputation).
If you are afraid people allowed to vote will miss question that should be closed, you may flag for diamond moderator's attention. But if it sits on the front page, just leave it be.
For the reasons from question you linked: They were never implemented. Don't know why. We use them as a rough guideline for freeform reason, and sadly that's all for now.