Online forms do not have that capability because they are in HTML format and not PDF (they also cannot be converted).
However, that is something you can advance to the technicians to work on next in regards to "Windows Ink" to create that capability to sign or write onto onscreen forms from web sites. In fact, nobody else does that even though they show that in tv advertisements, shows, and movies (which are all mostly fake). Some banks are adding that capability to their onscreen forms and again, forward that to the technicians to see if they can add that for any and all onscreen forms.
This is actually something that should have already been addressed in the Windows Insiders program but I guess, nobody thought of it for the future builds of Windows 10 and is a feature that 'may' be able to be added to the current version of Windows 10.