I failed to contribute to my HSA account when my company asked to enrolled for it. Is it still possible for me to contribute to it and also get the tax deduction ? If yes, I should be able to specify the amount in line # 25, right?
As long as your total doesn't exceed the per-year limit, you should be able to deposit after-tax money into your Health Savings Account. Contact the HSA administrator for details.
Note that unless your employer sets this up, you'd be sending them after-tax money, which goes in the same category as other non-reimbursed health expenses, so you may not get any immediate tax savings by doing this vs. just spending the money out of pocket.
However, once there us enough reserve money in your HSA for you to invest it in the same way a 401k can be invested, it will grow tax-free. So if you're putting in significantly more than you expect to withdraw any time soon, this may still be a worthwhile thing to do.
Definitely talk to HR about whether you can still get it set up pre-tax... though most employers don't allow midyear changes unless there has been a significant change in your family (new wife, new kids, that sort of thing).
Yes, you can make a direct HSA contribution and deduct it on your 1040 to see an immediate tax benefit for the tax year. You can even make a contribution for the previous tax year all the way through tax day of the following year. If you opt to do this be sure to contact your financial institution before making the contribution to make sure they apply it to the appropriate tax year.
Just be careful not to exceed the yearly caps. So long as you have had coverage all year, this is easy to do. If you've only had coverage for part of the year and it's not your first year with an HSA plan, you may need to do some math to make sure you aren't breaking the contribution rules.