Contacted via customer relations form chosing Miles and Smiles membership processes. Sent ID, screenshot of gold card from the app, and a screenshot of transaction activity from the last months. I also sent them electronic receipts for the tickets, which contain the flight details, my first and last name, and my Finnair membership number. Approved literally within one day with the usual challenge conditions: fly TK international in the next 4 months, accrue another 15k miles for an extra year.