I'll give it to you as javaScript (I don't us jQuery). This pattern, however, is complimented the same in virtually any language.
function yourOnClickEventHandler(some_input_reference) {
var anObject = document.getElementById('text');
anObject.value = anObject.value+"\n"+some_input_reference;
}
Now the argument to the function,
some_input_reference, is a bit vague. I happened to use it as though it were your added additional text BUT it may be, instead, a reference to something else.
For example, if your button objects had values then you could, for example, send the button's id as reference and used it as the argument for
document.getElementById(some_input_reference);
or, you may wish to us the object, itself "this" as an argument and then you can use
some_input_reference.value
and append it.
Lot's of routes, but the basic pattern is add/append/concatenate the new value to the current content of your textarea.