Yes, you can contribute to your HSA directly and indicate you are making a 2024 contribution until 4/15/25, as long as you had a high-deductible health plan (HDHP) and are contributing only up to the maximum for the year, depending on your plan, age, and the number of months you had the HDHP. Keep in mind that unlike payroll contributions, you won't get a break on FICA taxes.