I'm not familiar with Plesk so I'm unsure if you can edit the .htaccess file (although, I don't see why you wouldn't be able to). You can use something like this to exclude a path from the password requirement - https://stackoverflow.com/a/41289289
You would need to exclude
/index.php?app=core&module=system&controller=marketplace
Alternatively, you could set the test community offline instead of using a password prompt, it pretty much does the same thing.
Don't forget though, our license terms do provide allowance for one test installation 😉