Hi Sasika,
You can attach the OpenACS showCalendar function to a button and ask it to populate a textbox.
I'll use the recommended ad_form Tcl procedure to build the form:
ad_form -name calendar -form {
{start_date:text(text)}
{popup_cal:text(button)
{value " ... "}
{html {onClick "return showCalendar('start_date', 'ymd');"}}
}
}
...
For the showCalendar JS code, see /packages/acs-subsite/www/resources/core.js
For more info on ad_form, see this guide.