Amounts reported on Form W-2, Box 12 with a Code W - Employer contributions to your Health Savings Account will be transferred to Line 7, Column C of California Schedule CA to be added to income on Line 16 of California Form 540.
Per the California Instructions for Schedule CA (540):
Employer health savings account (HSA) contribution. Enter the amount of any employer HSA contribution from federal Form W-2, box 12, code W on line 7, column C.
To enter Box 12 amounts with Code W: