The broker/1099-B issuer does not know the basis, so you have to enter it. If you do not know the basis, you will need to enter $0, which will maximize what you pay in tax on that transaction. Cost basis is the amount you paid to acquire the security. Please see the help article below if you need help finding your stock's basis. The difference between the proceeds and the basis is the amount of taxable gain.
How to find a stock's cost basis
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