重定向WordPress页面而不更改URL或执行301

I have a URL like https://mydomain/pages/123_share but the corresponding WordPress page would be https://mydomain/pages/123/

How can I do this using .htaccess, WordPress wp-engine, etc.? Here, the problem is I cannot use 301 redirect. I need the content of that page served. That means whenever I type the first URL, I need to give the content of the second URL without passing additional HTTP headers or changing the URL.

How can I do that?
Thanks in advance.

If you have mod_proxy enabled you just use the same kind of rule as you would for an external redirect, but proxy it:

 RewriteRule ^pages/123_share$ https://mydomain/pages/123/ [NS,P]

The page should should have a <link rel=canonical … for SEO.