I have the same issue. I am trying to do something remotly on my computer from my phone, and it requires holding down W. Using the iOS on screen keyboard, I can't hold down W. I used the windows on screen keyboard, still can't hold W. I can hold W on my computer by holding click on the W key, but this won't work from teamviewer.
I found a workaround by using AutoHotKey.
For my purposes I needed to hold the V key down whilst clicking, so I need a long V press in combination with another key, rather than lots of tapping of the V key. My code for this is:
send (v down)
send (v up)
In English, this reads as:
When I press CTRL SHIFT F9 then hold down the V key, then click whilst it is pressed, then release the V key, then stop.
You can edit this for whatever key combination you care about.