Set custom primary domain

[straube]

The plugin sets the domain where WordPress was originally installed as the primary domain. Based on the following request, add a feature that allows users to flag another domain as the primary one -- internally called "original domain".

Is there any way to convert the canonical link/domain/url to a specific locality (say en-US) and set up an ‘x-default’ on a subdomain?

Currently, the plugin will add ‘x-default’ on the primary domain and there’s no way to stop it from doing that.

For other reasons, I am not able to change my primary site address (siteurl), which is at the apex domain. Otherwise I would have done that.

Reference: https://wordpress.org/support/topic/plugin-is-adding-trailing-slash-to-url/