Through my employer, I set a dollar limit, and they automatically calculate the amount per pay period based on the number of remaining pay periods. This sounds similar to yours.
However, this is only for contributions made by payroll deduction IN THE SAME YEAR.
If you want to make contributions in 2019 that are designated for 2018, you probably have to do that yourself by mailing a check to the HSA bank or making an electronic deposit on their web site. And as I indicated, this is NOT a normal contribution and it is VERY IMPORTANT to follow the procedures exactly to make sure the payment is designated for 2018. And it doesn't matter how many contributions you make in what amounts, as long as they are made by April 15, 2019 AND you follow the procedure to designate them as 2018 contributions for each and every payment that you want coded as 2018.
It is very likely that your employer will not designate 2019 payroll deductions as 2018 contributions. You need to do this yourself by contacting the HSA bank, using your after-tax or out of pocket money.
Then separately, contributions made by payroll deduction for 2019 made in 2019 will follow any rules your employer sets. If your employer wants to limit you to $291 per month, instead of $3500 in January and nothing the rest of the year, that is up to them.
