Remembering thus one "wolf of wallstreet" scene you should create a need in the head of your dialogue partner in order to be convincing.

Like "would you please give me the cash in your wallet? I will keep it for you, trust me..." 
If the person has already bought Bitcoin on Binance, ask the same but swap "cash" with Bitcoin. 

If the answer is "No!", it should be easier now to argue for self custody.