Based on @jdm2112 response, I did the following:
add_filter('gform_paypalpaymentspro_args_before_payment','add_donation_comment', 10, 2 );
function add_donation_comment( $args, $form_id ) { // do not edit $args or $form_id
global $blog_id; // Make this available to the function
// Apply filter only if blog id is 2
if ( 2 == $blog_id ) {
// apply to form 1 only
if ( 1 == $form_id ) { // Replace '1' with your form id
$args["COMMENT1"] = 'Field Project'; // Comment1 is the arg that paypal pro uses.
}
// apply to form 2 only
if ( 2 == $form_id ) { // Replace '2' with your form id
$args["COMMENT1"] = 'Help Us Grow';
}
// always return the args, even if not changing anything
return $args;
}
else {
// always return the args, even if not changing anything
return $args;
}