I received NJ state disability payments though my employer and have not received a W2 form. Can I enter this payment information in Box 12a?

[WillK]

That depends on how you paid the premiums for the disability. If the employer shared in the cost for the premiums, then yes, the short term disability is taxable. The payments (claim payments) will be taxable based on the proportion the employer paid towards your premiums, and the portion paid by you if you used after tax payroll deductions. If you paid for the premiums entirely by yourself using after-tax dollars, then the payments (claim payments) are not taxable.  

If the employer did share in the cost paid for the premium then you would need a W-2 from the employer to show what was taxable. Please contact your employer and request that information. The payments made to you would be reported in box 1 since it is taxable wages. 

If your employer did not share in the premium cost then the benefits are not taxable to you and do not need to be reported.