Under Construction – Send to external link?

Is there any way (plugin or any other way) to have a “Under Construction” page that sends the visitor to an external link?

Here is why:
Today we have a HTML based site. This needs to be accessed by visitors during construction of the new WP-based site.
The construction might take a while.

So I am thinking of having https://www.example.com redirect to https://www.example.com/example during this period. (Existing site is no problem moving to a subdirectory as is).

But whle building the WP-site i need the features that “Under Construction” usually gives (bur instead of showing under construction-page sending to another site).
In other word beeing able to build and adjust the site and testing it etc.

/Jocce