Sorry you can't apply the Desktop purchase to the online version. They are separate things and don't talk or sync to each other. You would have to get a refund for the Desktop CD/Download program and pay for online separately.
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.
