Derek is correct. But it would be great if you could take a moment and vote for this feature on User Voice, which is checked by Adobe when deciding which new features to add. Thanks! https://indesign.uservoice.com/forums/601021-adobe-indesign-feature-requests/suggestions/31518886-al...
Thanks for reaching out. Derek has suggested you correctly. Publish Online documents are hosted on Adobe servers. Each published document is assigned its own unique URL. Only person with the URL will be able to access it(Just like many file sharing services). You can not protect it with a password though. Please find more information on https://helpx.adobe.com/indesign/publish-online-faq.html
You may up-vote the UserVoice link shared by Diane. This forum is monitored by product team and they might look into it.
Hope it helps.
Just tried to use the (new) preview share send a link and it asked people for a password, which I can't find an option to create a password in the first place. I wish it was slightly easier for people to understand on their end what to do since I can't see what they get.