Use your preferred Macro software, I use Razer synapse as I have Razer peripherals but any program should work.
This is a better option than other ways I've seen it done as it's not operating system reliant and you can use it for any key not just the mouse keys.


1. Hit Record
2. Press Left Mouse
3. Hit stop
4. Delete the upstroke entry
5. Save your macro
6. Bind it to a key and select 'play once'

It should look like this (more or less)
⠀

Now all you have to do is press your macro key and it will hold down the mouse button,
To stop the macro all you have to do is click your left mouse button as it will send the upstroke command when you let go, that's it your done!


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Xoxo Sworn.