Since quite a while it’s been bugging me, that Defold does not properly group into the (Windows) Taskbar 
Opening Defold just launches another process which gets spawned at the end of your taskbar, pinning the running (Editor) process is also not doable by default. I’ve found no information regarding this on the web, an old Forum post already mentioned this - but no solution was found at the time 
One quick little workaround approach would be simply editing a Registry Key to achieve “normal” grouping behavior for Defold! Step by step instructions below:
(Please do not make any random changes to your system, verify first what something does!)
Editing the Registry
Registry Path:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileAssociation
Open “regedit” & navigate to the path in the registry. Open the Key named “HostApps”, remove “JAVA.EXE;” & “JAVAW.EXE;” mentions from it, keep everything else as it is! (Also make sure to catch the semi-colon, no double or missings!
Re-log into your Userprofile to finish up! (IMPORTANT)
Making a backup of the Key and/or setting the location as a favorite in regedit for future redoing is/could be advised! 
Pinning Defold:
Try opening up Defold, it should now spawn a seperate process which you can pin - it might look a little off from the beginning, but we’ll get to that! 
Simply right click, pin & you’re almost done! 
All new instances of Defold will open into this pinned shortcut! 
Customizing your new Icon
Taskbar Pinned Apps Path:
%AppData%\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar
Copy the path & paste it either into your run prompt by pressing Win+R or straight into your explorer.
All your pinned items should be displayed here, find the OpenJDK, right click it - change its icon & name!
(Defold.exe is a viable icon source!)
Re-log into your Userprofile after making changes, restarting explorer may (will) also help (I guess/hope)
If it’s not updating your icon, name or whatever, simply try (un)pinning again
Maybe it creates a new icon or accepts your one?
BUT THINK OF RE-LOGGING (RESTARTING EXPLORER)!!
Customize it to your liking!
You now have a working Launcher & grouping Editor in your Taskbar!
I’ve tested this (in addition to my main install) on a newly installed VM, with & without Java installed!
