Generally "if you're good at something don't do it for free" and without knowing details of your situation it's hard to advise. I'd be cautious as working for free typically devalues the work (red flag #1). It's the same as being to eager to make a new friend or find a girlfriend; goes back to being "gatekeeper" in that if you spend your time/wisdom etc too freely then it devalues the commodity.
Additionally an employer asking you to work for free is a red flag (#2). Mentioning a startup environment is red flag #3. Be careful - startups have this way of not paying you (even once hired), asking for more "free/after hours" work, micromanaging, or being led by delusional founders.
Giving benefit of the doubt to all of that... it's a long read but I made a 12k word SE starter series (50m video summary). Basically cater to their interests and beliefs and keep in mind that our world is often not based on fact, merit, or objective reality.. ex: If they are narcissistic don't play up your own skills but rather focus on how them mentoring you has lead to your success. They are more likely to believe your success is based on them and not you if they are a narcissist. Etc etc
Also - this prior salary post. Essentially:
Anchoring bias. Start the conversation with a high number, mention "fairness" since it's used to knock people back emotionally. Bring it up first. "I honestly want to be fair here so please stop me the second you think I'm not" then segway into selling yourself. This way it's harder for them to say "that's not a fair number" and you can counter the question with the sales pitch if it does come up.
Find common ground and be likable. "Master of both worlds" as Dan Harmon puts it - sell not only your skills but personality. Don't be a pushover though. Instead aim for "tough but likable" - they want someone "tough" to fight for them and represent their interests/goals just don't overdo it.
Salary isn't the only commodity. Think vacation/sick time, training/expenses paid, school reimbursement etc. Startups will try selling you on company stock as you're "getting in on the ground floor/early" etc but it's risky. Understand the risks and act accordingly (best bet is to diversify).
You can do research and bring up salaries for that job in your city and places with similar cost of living (COL). If you try negotiating a "raise down the line" get that shit in writing and be prepared to walk away if it falls through.