There is no code. Box 14 is a "memo" field, nothing you enter in box 14 has any force under Federal tax law or the instructions for filling out form W-2 for employees. Some states place some state-specific items in box 14, and turbotax can recognize some of these items. But there is no official code for clergy housing allowance in box 14 (unless your state requires it) and a clergy housing allowance in box 14 is not automatically registered by Turbotax. Your church may use box 14 to inform you of the value of your housing, but they are not required to. When entering a W-2 with clergy wages, you must check the box for "religious wages" on the page of special circumstances, then enter your housing allowance on the next screen when asked. Turbotax will also ask for your qualified housing expenses. If your housing allowance is more than your qualified expenses, the difference is added to your taxable income.
The value of your housing is either the fair market value of your parsonage or a cash housing allowance, you have to enter it either way.