Ana here, from the Shopify guru team! :)
Were you following the instructions in this guide? https://help.shopify.com/themes/customization/communication/ask-how-customers-heard-about-your-store
If not, give that a go.
There is a list of themes that this customization won't support, but I've just tested it on my end and these instructions do work for the Minimal theme.
I can follow-up with an email if you're having problems getting this set up properly - let me know and I'd be happy to!
I'd probably avoid doing this personally. All ressearch into conversion rates suggest that adding any additional fields into the checkout process can reduce conversions. You want the shopper to have as quick a process as possible.
Another way to get this info would be in a follow up email, or shipping notification email. Perhaps include the question in there instead?
If you do add this into the checkout make it optional, rather than mandatory.
Thanks for your reply. Yes, I followed these instructions and it looked good until I had a customer try to check out. The checkout portion did not work. I spoke with the Shopify help group and they told me it was because minimal was not a supported theme. I'm trying to see if there is another way to accomplish this.