MailPoet subscribe checkbox not shown in WP Members register form

I am new to WordPress and trying to send newsletters to subscribed users when new posts are created.

I use WP Members as membership management plugin. The problem I face with it is that the MailPoet subscribe checkbox is not shown in the register form although the according option is ticked in the MailPoet settings. The checkbox is shown in the default WordPress register form but not the one of WP Members.
What am I doing wrong?