Hey Gal, didn't do it with Zendesk but with Jira, Shopify, and Wordpress.
Some most obvious ones that come to mind:
1. join the Zendesk marketplace
2. take care of SEO (cover keywords like "Zendesk + [your use case]" or just "[your use case]" in general
3. Google Ads can be quite cheap for long-tail terms (even brand-related), you can even use the Zendesk marketplace app page as a landing page
4. outreach to blogs related to your use case that could mention your new solution
5. in-app & email list announcement
6. social media announcements
More might depend on what exactly your product does and who your customer is. Let me know if I can clarify anything here