Quick Question Regarding Message Popup

Hi there,

I have installed the plugin and using short code to display the "Message Me" button in the loop,

<?php echo do_shortcode('[pm_user user_id="'.$organizerID.'" text="Request a Quote"]';?>

all working well, although I wanted to customise the popup form which appears when you click the button. Could you let me know which file to edit to customise the form please.

Please see the attached screenshot.

Thanks for your help