Han, there's no right or wrong answer here, it's all about what feels right to you. You are never required to disclose, and you can do so at any point, whether you're just interviewing, accepting an offer, or you've been with the company for 10 years. If you've decided you'd like to disclose, you can share with HR, or your manager if that's more comfortable for you. I've always gone with my manager because they're the person I work with every day, and it tends to open up a discussion about how we can make work more comfortable.
Do you have any suggestions for resources for those without access to healthcare to get help for PTSD?