Canapin
(Coin-coin le Canapin)
11
Certainly 
Each time you do any action in Discourse, you can check the network tab of your browser dev tools to see the query that was made.
For example, here I save the profile of a user:
You see the request URL, mentioning the username, and the type of request (PUT).
On the Payload tab, you see the relevant data:

The custom fields are listed as user_fields[X]
where X is the number of the custom field.
Edit: I saw that you figured it out on dev, so, just to crosslink… 
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