Magento 2.2.

I have a theme:

/app/design/frontend/Companyname/Newtheme/ 

I would like to override this PHTML file:

/vendor/magento/module-customer/view/frontend/templates/account/authentication-popup.phtml 

What is the correct directory structure in my theme’s folder to put my new file?

I have tried:

/app/design/frontend/Companyname/Newtheme/Magento_Customer/view/frontend/templates/account/authentication-popup.phtml 

and

/app/design/frontend/Companyname/Newtheme/Magento_Theme/view/frontend/templates/account/authentication-popup.phtml 

and

/app/design/frontend/Companyname/Newtheme/Magento_Theme/templates/account/authentication-popup.phtml 

p.s. I know this is a really basic question, I am just not understanding the documentation and many examples online. Not sure why.