A Photoshop Action that fill your line art on a separate layer with "foreground color" with a single button.

To use Action, go to Photoshop>Windows and make sure there is a tick beside Action. Then find and click on the button with a "PLAY" shape. At the top right of the "Action" window, there's a triangle pointing downward with a few lines on the right. Click it and click on "Load Actions". Find the .atn file and load it in. To assign a hotkey to an action, double click on the action and change the "Function Key" to your preferred Hotkey.