get value from jprompt before calling rowcommand event of gridview in asp.net c#

If you want get data at clientside you can use onClientClick propery of control that is causing postback/rowcommand

I guess you are using jpromt from http://labs.abeautifulsite.net/archived/jquery-alerts/demo/[^]

So you can have something like

jPrompt('Type something:', 'Prefilled value', 'Prompt Dialog', function(r) {
    if( r ) 
    {
         $('#MyhiddenField').val(r);
    }
});