There is no such thing as a password for a public IP, a public IP is just a resource assigning IP's to a network interface, nothing more.
If you are hosting an application in Azure it is up to you to make sure this is secure. Ideally, this would be done through authentication at the application layer, to prevent users from being able to do anything in the application without authenticating. If your application does not provide this then you may want to take a closer look at your application and whether it is fit for purpose.
If application level authentication is not possible then you could look at adding authentication at the application server level, be this Apache, IIS, Tomcat etc. You would need to look at the appropriate documentation for your application server.