If you contribute now (before April 15th) to your HSA for 2018, then you must report it on your 2018 return. Yes, if you have already filed, this means that you would need to amend your 2018 return. You do not have to file the amended return before April 15th, you just need to make the contribution before April 15th.
No, you cannot claim this as a deduction on your 2019 return, unless, of course, you told the HSA custodian it was actually for 2019 (which I gather is not your intent).
If you are positive that this will not result in an excess contribution, then go ahead and make the contribution now (before April 15th). You can amend later at your leisure, so long as your original return has been processed (like, after you received your refund).
You should be able to amend now with your desktop software (make sure it is up-to-date). But to be clear, you do not have to amend by April 15th.
Please see this TurboTax FAQ for how to amend your return with TurboTax - https://ttlc.intuit.com/replies/3288565
