Lost access to wallet, can I tag transactions 'lost'?

Hi there, I’m in the Uk, used to send bitcoins between my wallets and can’t access some of them and used Btc-e exchange in the past what is not available any more. Bitcoins coming from them have 0 cost basis no doubt but can I tag transactions when I sent Btc into them as ‘lost’ or do I have to pay tax on these gains even if I had no gains at all because after sent them to an other wallet again? I hope my explanation is understandable. Thank you for answers

Hi, bitcoins coming from unknown wallets will have the fair market value as their cost basis. You could mark the withdrawals as Lost but in this case Koinly wont be able to keep track of the costs associated with the coins that were sent into this lost wallet.

An alternative:
You could create a custom wallet called “lost” and change all withdrawals to “transfers” and all deposits coming from this lost wallet to transfers too. This would carry over all your cost-basis.

Thank you so much for the answer, will do that!