Social media platforms can be used to provide effective support for users with mental health issues. Self-disclosure about mental health on these platforms can lead to social support and community building. However, there are also inherent risks of stigmatization and misuse of platform features for maladaptive purposes, such as to promote eating disorders. To address these issues, it is essential to understand the reasoning and mechanisms of self-disclosure regarding mental health issues. However, this can be quite challenging, given the flexibility and complexity of user interactions on these platforms. For example, users can now share content not only privately or publicly, but also permanently or temporarily. Our initial inquiry with Instagram showed a tendency to co-opt mental health hashtags to increase post popularity. Based on our initial findings, we propose a participatory design study to better understand the motivations and challenges faced by users who disclose mental health issues on Instagram. Findings from this work can guide future designs to improve user experience and provide effective social support.