Hi Abhishek,
Have you tried using the preSubmit event? I have pasted the description of the event from Designer's on-line help below.
preSubmit event
Event type: DOM oriented events
This event triggers whenever a form submits data to the host via the HTTP protocol. At this point the data is organized into a data set, but has not been sent to the host. Scripts associated with this event have the chance to examine and alter the data prior to the form submission. If the script is set to execute at the server, the form sends the data to the server with an indication that it should run the script before performing any additional processing.
The preSubmit event applies only to the Form DOM. Note that the preSubmit event does not distinguish between submissions initiated by different button pushes or to different URLs. Any script that needs to make these distinctions must include code to find out what button was pushed. In general, preSubmit is analogous to preSave and serves a similar purpose.
See also
preSave event
Denver
Adobe Enterprise Support