Are you sure it's for a IRA? Maybe it's a ROTH 401k? Either way ROTH contributions are not deductible. If you contribute at work you only enter it from your W2 nowhere else. You contribute to an IRA outside of work with your own money.
A qualified workplace plan, whether it is a 401(k) or 403(b) or something else, is not an IRA, they are similar in purpose, but they are controlled by completely different sections of the law and many of the rules governing contributions and distributions are different. Any contributions to a workplace retirement plan are already reported on your W-2 and you do not report them again separately.