If you made an HSA contribution for 2018 that you want to include on a return, you will need to report it on a 2018 tax return. Therefore, if you have already filed your 2018 return, you will need to file an amended return (Form 1040X).
See the FAQ below for links of how to amend your 2018 tax return:
https://ttlc.intuit.com/replies/7276605