How would you tell someone you're dating that you have an extremely rare invisible disability? Would you mention it immediately or wait?

I have New Daily Persistent Headache. I've had a tension headache for two years straight. I'm at a 3-4/10 most days on a pain scale. Worried about a date coming up and wondering how to tell her about my pain. I can still work and do anything a normal person can do.

How would you tell someone you're dating that you have an extremely rare invisible disability? Would you mention it immediately or wait?
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