changing comment textarea placeholder

I want to moddify the default text in the comment textarea to be the same as the comment label, though I can not find out how. Changing comment_field in the args section didn’t help (I have succesfully changed ‘logged in as’ though).

I have solved it (as a test) by changing the following function in comment_functions.php:

public function comment_field_label_filter($comment_html) {
		return preg_replace('/(<label.*>)(.*)(<\/label>)/i', '${1}'.$this->options->get('cgb_form_comment_label').'${3}', $comment_html, 1);
	}

to:

public function comment_field_label_filter($comment_html) {
		$comment_html = preg_replace('/(placeholder=")(.*)(")/i', '${1}'.$this->options->get('cgb_form_comment_label').'${3}', $comment_html, 1);
		return preg_replace('/(<label.*>)(.*)(<\/label>)/i', '${1}'.$this->options->get('cgb_form_comment_label').'${3}', $comment_html, 1);
	}

This is, ofcourse, ugly and will break at the next plugin update.

Is there a way to simulate this without editing the plugin directly? I didn’t get it to work with a filter yet. Editing my functions.php with a hardcoded replacement for the placeholder would be okay for me, but I preferably either copy the text from the label or have another input field for the placeholder on the plugin page.

https://ww.wp.xz.cn/plugins/comment-guestbook/