You can't move your prior Desktop returns to your online account. You CAN transfer 2018 from your computer to Online 2019.
How to transfer into the Online version, expand the blue link if you used the CD/download last year........
Oh, by the way,
If you used the Desktop program before you may not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2019 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.