Xara (with an 'X')

Xara Web Designer or Designer Pro have no password function, mostly because they are not HTML editors but also becuase a function such as this is best done at the server (your host). This is why Word Press (a server-side CMS system) has this function available.
The best suggestion I can make at this point is to set up a .htaccess and .htpasswd for that folder via your hosting cpanel.

This will result in a UN/PW prompt similar to the one shown below.
(Some of my wholesale suppliers use this exact method)
Click image for larger version. 

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