ИР, я нашёл!

function insertAtCursor(myField, myValue) {
//IE support
if (document.selection) {
myField.focus();
sel = document.selection.createRange();
sel.text = myValue;
}
//MOZILLA/NETSCAPE support
else if (myField.selectionStart || myField.selectionStart == ‘0′) {
var startPos = myField.selectionStart;
var endPos = myField.selectionEnd;
myField.value = myField.value.substring(0, startPos)
+ myValue
+ myField.value.substring(endPos, myField.value.length);
} else {
myField.value += myValue;
}
}
// calling the function
insertAtCursor(document.formName.fieldName, ‘this value’);