I had a chat with a manager but haven’t maintained the relationship. How can I convert it to a referral?

Potential solution:

If it were me, I would forget about the referral. I would simply find a way to add a bit of value to them. When you chatted with them / researched their profile, did you identify any of their interests or achievements? For example, if they enjoy xyz, you could share a podcast episode of XYZ. Basically a “maintaining message”… then they might ask how you are etc. and can warm into talking about jobs