Does this work with Ninja Forms?

WP-Members allows you to choose to display or to not display both the login and registration forms in place of blocked content. These are the check boxes for “Show login form” and “Show registration form” in the plugin’s main options. If you’re using some other process, uncheck those; although I would recommend leaving login enabled since that makes it easy for the user to log in immediately if they land on a page that is blocked and they are not already logged in.

The settings for “Pages” in the main options tab allow you to select from any “page” (not posts or custom post types). These do not have to be pages that use the WP-Members forms – they can be whatever pages you need them to be. If the page you need is not listed, select the option for custom URL (which is the last one in the list) and enter that URL in the input provided when that option is selected.

The initial setup video may be helpful in this regard (although some of it won’t be directly relevant to your situation):
https://rocketgeek.com/plugins/wp-members/docs/videos/