The Last Month Rule states that if you are covered by an HSA eligible health plan on the first day of the last month of a given year, you are considered an eligible individual for the entire year. This gives you the option to contribute the entire year’s contribution limit to your HSA, which is more than you would be allowed otherwise (pro rata by month).
The Testing Period states that if you use the Last Month Rule, you must remain an eligible individual (covered by HDHP) for the next 12 months, so through December 1st of the following year. If you fail to remain an eligible individual (change insurance plans, lose insurance plan, receive other health coverage) during that time, any “excess” contributions you made as a result of using the Last Month Rule will be taxed and penalized.
Perhaps the safest strategy is waiting until the following year (but prior to April 15th) to make a Prior Year Contribution. This ensures you do not over contribute during a period and have to declare and pay tax on the excess
i'am assuming you are not married and won't be married on 12/31/19. what spouse has in the way of health insurance and HSA could affect what you can contribute
as single you can contribute $3,500 for 2019 reduced by employer contributions. so you need to either know this amount so you don't over contribute or if permissible make the contribution for balance for 2019 by 4/15/2020 or whatever your plan permits.
in theory say employer contributes max of 750 then you can contribute up to $2,750, but if the 750 is prorated you can contribute more, if you want. there is no requirement to max out the HSA contribution
