A plugin creates a set of pages and a template. There must be some method to assign the template to one of the pages. I’ve attempted to adapt the method shown at filter reference
template_include without success.
The page slug is
member-register, the template file name is
Some of the things that don’t work include:
locate_template(): I suspect that because the template was created by a plugin it is not found by a function that appears to only search themes.
is_page(): If given the argument of the page’s title, slug or id returns false.
Read more here: Code to add template to page