Can I adjust the amount in box 1a of a 1099-K from Ticketmaster since they put more than the amount you were paid for your ticket sales?

The gross payment amount (Box 1a) on Form 1099-K reports the total, or gross, dollar amount of reportable payment transactions. It doesn't include adjustments for fees, credits, refunds, shipping, cash equivalents or discounts. Those items are not income. You can deduct those items from the gross amount when including the income on your tax return.

For more information check :Form 1099-K FAQs