Please check with using val() method to set or getting the value for the textbox rather than using text() method as below :-
NOTE : This text() method works on both HTML and XML documents. Cannot be used on input elements. For input field text use the val attribute.
$(function () {
$('.tag').click(function () {
var tag = this.innerHTML;
var smstext = $('#<%=txtEnquiryText.ClientID %>').val();
smstext = smstext + tag;
$('#<%=txtEnquiryText.ClientID %>').val(smstext);
});
});
Hope this will be of help to you.