Using Aliases inside Admin Panel for fields
A good approach would be to use the UrlHelper class it has a function UrlHelper::url() that is available in Twig and will return the correct URL for a certain path. So you could ask your client to not insert the entire URL, only the path -&gt; instead of example.com/entries -&gt; entries or you can include a custom Twig function/behavior that strips the domain before you use the {{ url('entries') }} function.
Anyway: to be honest I would create a different approach in the first place. Creating static URLs in a text field is usually not a good thing. I'm always a fan of the

protect clients against themselves

philosophy so you may want to consider to create a custom field type with autosuggestion or something. SEO wise it could be a bad idea if you clients are able to insert whatever they want and you output too many dead links. Maybe a Relation field could be something for you?