You should attach an onchange-eventhandler to the textbox. Then compare the last seven characters, and if they're not "abc.com", add that string at the end.
$('#textbox').change(function() {
if ($(this).val().substr(-7, 7) != 'abc.com') {
$(this).val($(this).val() + 'abc.com');
}
});
Edit: Possibly the onchange-event may interfere with the user input. In that case, use the blur() event.
Also, if the length of the 'abc.com' string may differ, you can probably save 'abc.com' in a variable and use (-variable.length, variable.length) to select the substring.
And if you don't want the user to be able to edit the 'abc.com' part, remove it on focus() event, and re-add it on blur() event.