There should be two cables running out of the alternator.
Big thick one (usually red) and a smaller thin one which goes to the charge warning lamp.
If your relay needs an excitation current then the best way would be to crimp a round connector on it and attach it to the charge lamp connector on the back of the alternator.
However, most newer alternators tend to use less reliable "spade" connections.
In that case, you may need to use a Skotch Loc connector and attach it that way.
Another however, some alternators have two small gauge wires, one of which is a voltage sensor wire. It's job is simply to tell if the alternator needs to up the current if it's under heavy load.
You will need to see which your car has.