Highrez

So my mouse has two extra buttons on its left side, which are displayed as mouse button 4 and mouse button 5 in XMBC. What I want to do is to reassign button 5's function so that when I click it, it will hold down the left mouse button for a specific amount of time (500ms, 1s,...). How can I do that, or is it even possible?

Select simulated keystrokes
Then use method one (in the dropdown) with the code "{HOLDMS:500}{LMB}" (adjusting 500 as necessary)