If there is nothing else on the W2 do not enter the form W2. Technically since you were not an employee this probably should have been reported on form 1099-MISC not a W2. So you are going to enter as an HSA contribution made on your behalf.
Go to the "Deductions and Credits" section and scroll down to the "Medical" topic and use the "HSA... contributions" topic to enter the contribution amount. TurboTax will ask if you made contributions to an HSA. Answer "Yes" and you will arrive at page that asks for the contribution amounts and should show zero for the W2 box 12 amount. You will enter the amount from box 12 coded W along with any contributions you personally made in the box that says "Contributions you personally made or others made for you."
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