Yes, you can still contribute to your HSA. This excerpt is from last year's Pub 969, but is still valid. Change the date from 2018 to 2019 and from 2019 to 2020.
Form 8889.
Report all contributions to your HSA on Form 8889 and file it with your Form 1040 or Form 1040NR. You should include all contributions made for 2018, including those made from January 1, 2019, through April 15, 2019, that are designated for 2018. Contributions made by your employer and qualified HSA funding distributions are also shown on the form.