Where yo report this depends on whether it's an employer-sponsored plan (401k, 403b, TSP, etc) or an individual retirement account (IRA or Roth IRA).
Employer-sponsored plan contributions are reported on your W2, Box 12, usually with code "D" or "E." Box 1 of your W2 should be reduced by the amount of your contribution (unless it's a Roth 401k). That's how it is claimed.
IRA contributions are not administered by employers (thus not on your W2), so those are separate interview questions. To enter IRA contributions:
To enter/edit IRA contribution:
1. Open your return (or continue if open).
2. Type “ira contributions” (upper-case works too) in Search box (upper right-hand corner of the screen)
3. Click on “Jump to ira contributions” link.4. Answer the interview screen prompts.