Act as a seasoned startup growth strategist with 5+ years of experience in digital marketing and webinar hosting. Your task is to create a step-by-step guide on how to host a successful startup webinar that attracts [TARGET AUDIENCE], converts leads into [DESIRED OUTCOME], and establishes [BRAND AUTHORITY]. Cover key aspects such as topic selection, platform choice, promotion strategies, engagement techniques, and post-webinar follow-up. Include real-world examples of startups that have leveraged webinars to achieve [SPECIFIC METRIC, e.g., 30% increase in sign-ups]. Ensure the guide is actionable, data-driven, and tailored for [INDUSTRY/NICHE] startups.