How do I show the source of the funds if I have exchanged cryptocurrencies on a platform from which no transaction confirmation can be generated?

In such a situation, we can accept history in the form of screenshots showing the exchange transaction.

It is important that these screenshots show an identifier that allows the owner to be identified, e.g. an e-mail address, login or name that also appears in our system, so that the data presented can be linked to the person concerned.