If you were HSA-eligible in 2018 and have not already contributed the maximum for 2018 to another HSA based on the type of HDHP insurance coverage you had in 2018, you have until April 15, 2019 to open an HSA and contribute for 2018.
A personal HSA contribution deposited by you to the HSA (not through your employer), is deductible on line 12, Schedule 1 (Form 1040). TurboTax will automatically generate this deduction when you enter a permissible personal HSA contribution.
You can keep this HSA (it belongs to you, not any employer), and use the funds for qualified medical expenses for any expenses that were incurred after the HSA was created (in 2019).
Once created, you should definitely keep the HSA around and use it until the funds are exhausted. Note that if you ever have HDHP insurance again, you will be able to resume contributing to the HSA (if you don't close it), and you can even pay for medical expenses that you had during the period you could not contribute, because it does not matter when you reimburse yourself from an HSA, so long as the event occurred after your HSA was created.
Of course, you will want to check the fees of keeping an empty HSA at the custodian, because the fees may eat you up or the custodian may just decide to close the account. Talk to them.
[Edited 4/1/2020 11:35 am CDT - updated for 2019]
