HSA contribution for 2018

[DenCo]

Hello,
in 2018 my employer contributed to my HSA $900, while I contributed none.
Now that I realize the benefits of HSA contribution, I would like to still catch up for 2018 before I max out 2019. From the articles I read, I understand I can do that up until April 2019 filing date, and still write off the contribution for 2018 income tax. My questions are:
1. Is there any special form I need to fill out? My HSA has a contribute link to transfer money from your checking account, but since it's already 2019, how do I make it officially a "2018 contribution"? Or is this for me to note in case I ever get audited?
2. Will this affect my 2019 contribution? I already setup a regular paycheck deduction to add up to 3500 by end of year.

[MDM]

Hello,
in 2018 my employer contributed to my HSA $900, while I contributed none.
Now that I realize the benefits of HSA contribution, I would like to still catch up for 2018 before I max out 2019. From the articles I read, I understand I can do that up until April 2019 filing date, and still write off the contribution for 2018 income tax. My questions are:
1. Is there any special form I need to fill out? My HSA has a contribute link to transfer money from your checking account, but since it's already 2019, how do I make it officially a "2018 contribution"? Or is this for me to note in case I ever get audited?

The HSA administrator should ask the designated year for any contribution made between 1/1 and 4/15.

2. Will this affect my 2019 contribution?

No, if you designate this lump sum for 2018.

I already setup a regular paycheck deduction to add up to 3500 by end of year.

Careful: the total of your and your employer's contributions can't exceed the pertinent maximum.

[DenCo]

Thank you. That clears it up.