First, this doesn't sound like an interview but rather a coffee chat or informational call.
Second: Please don't directly ask for a referral.
How to ask indirectly for a referral
You essentially have 2 chances to "hint" at a referral.
So, near the end of the call, you can say "Thank you so much for your time. The company seems great and I will definitely be applying. Do you have any tips/advice for me?"
The 2nd "hint" comes a few days before you apply. Email them and say "Thanks again for meeting with me x weeks/months ago. I just wanted to let you know that I'm applying in a few days and appreciate all your time!"
Now, in terms of the actual chat, I have a few additional tips.
Ask For Yourself
While a coffee chat is a way for you to make a good impression, please remember that it's also about you making sure this job + path are right for you!
To prepare, think about things you'd like to learn about the company. Ask about your person's experience at the firm, where they've come from, if they have any advice for you etc. Be inquisitive and interested in what they have to say, and feel free to ask thoughtful follow-up questions.
Have Your Pitch Ready
In terms of what they'll ask you, make sure you have your 30 second pitch prepared! Who are you in a nutshell and why are you interested in Mckinsey and strategy consulting?
Ask About Them
People love to talk about themselves. Just ask them what they've worked on recently or what their journey at Mckinsey has been like. Importantly, do your research beforehand - find out what they've done and try and pick something from their past that looks interesting to you. Tailor your questions just like they tailor their FIT questions to your resume.
"So, how did you end up in the Industrial Goods sector?" "I saw you worked on a big digital transformation for a financial services company - I experienced one myself and it seemed so hard to move such a big organisation. What did you see as the major challenges and breakthrough moments?"
Here are some other great related Q&As