add_action( 'frm_entries_footer_scripts', 'add_footer_scripts', 20, 2 );
function add_footer_scripts( $fields, $form ) {
	if ( $form->id != 5 ) return; // Change 5 to the ID of your form.
	?>
	//add Javascript here
	console.log( 'hello' );
	<?php
}

Use this code to limit the maximum number of characters accepted in a field.

add_action( 'frm_entries_footer_scripts', 'set_custom_limit', 20, 2 );
function set_custom_limit( $fields, $form ) {
	if ( $form->id != 5 ) return; //change 5 to the ID of your form
	?>
	jQuery( document ).ready( function( $ ) {
		$( '#field_FIELDKEYHERE' ).change( function() {
			if ( $(this).val().length > 15 ) {
				$(this).val( $(this).val().substr( 0, 15 ) );
			}
		});
	});
	<?php
}