In windows you can use Autohotkey to do this.
Autohotkey accompanied with the script below, will essentially do a mouse click every time you press
Download the scripthere or alternatively copy it from below.
Once you have AHK installed, double click the script and it'll start running right away.
To close the script: Right-click the script in the system tray and choose to
Exit the script
Note that this essentially overrides your normal b key behavior, so be sure to exit the script when you don't need it anymore. Also, you could set multiple key combinations instead of a single key.
...and here's the code, if you don't feel like downloading the file, or it's not available anymore.
b:: Click down
b Up:: Click up
Copy the code and paste it into notepad or something and save it as
As a bonus:
If you want to click a key to activate it and click a key again to deactivate it, you could do it with this code:
if toggle := !toggle