Creating a MACRO that repeats when held

I play a lot of online games, and some of them do not allow you to use certain keys to do things. BTT has been my workaround. I am very new to the app and am seeking an option that holds down the action when inputted. In my instance, I use the shift key as my input and in return, the mouse clicks. I was wondering if there was a setting that holds down the click if I hold down my shift key. Thanks!