You should make an estimated payment to cover the estimated taxes in the quarter you liquidate the stock and realize the gain.
What are estimated taxes?
https://ttlc.intuit.com/replies/3301588
Do I need to make estimated tax
payments to the IRS?
https://ttlc.intuit.com/replies/3301891
How do I make estimated tax
payments?
https://ttlc.intuit.com/replies/3301258
