In my case, I followed @Chris Hayes, method of finding the WMClass and update the file, see below for an example. I referred other *.desktop files in
/usr/share/applications and found that Ubuntu 20.04 is using "StartupWMClass" as the variable and NOT "WMClass". I could not find the Icon for this application so I had to download from the internet a jpeg file icon from Google Image search, save it in the path and assign to the variable "Icon".
Please note, I believe this field "StartupWMClass" is case sensitive
Comment=View pdf documents
StartupWMClass=PDF Studio Viewer