A few ways to do this.
First is to attack the target yourself with your fleet wing on "match my target" orders. They will attack your current select target.
If you want more control, use "fire on my command" in the fleet control panel and click the fire now button when you have them in arc.
Fighters and PFs are generally the same way, except you use the shuttlebay panel to issue commands to them.
The fleet control panel is the button with two overhead ship silhouettes. As far as I know, you can't shortcut key it.
If you want them attacking something other than what your attacking, i'd probably target the desired ship, select 'match my target,' wait until they actually engage that target, then set them to 'fire at will' and leave them to it. It'll give a spotty performance, but it has a chance of working.