You will typically have to report income and pay taxes when the RSU vests and you receive actual stock shares. At that point, you have to report income based on the fair market value of the stock. Since RSU's are essentially compensation, you'll usually see it reported automatically on your W-2, included in Box 1 wages.
The value of the exercised shares may be noted in Box 14.
Here's more detailed info on RSU's and Taxes.
@jatellol
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