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add_filter( 'gform_submit_button_3', 'form_submit_button', 10, 2 );
function form_submit_button( $button, $form ) {
	$img = get_template_directory_uri().'/library/images/arrow_right_black.svg';
    return "<button class='button gform_button' id='gform_submit_button_{$form['id']}'>Register <img src='{$img}'/></button>";
}