כאשר המשתמש מקיש על ENTER.

כך:

// get the input element as DOM object.
var input = document.getElementById("input-id");

// create the event listener
input.onkeypress = function(e){
e = e?e:event; // fix browser compatibility
if(e.charCode==13){  // check if the user press on [Enter] key
   // call to function
   alert("enter is pressed");
}
}