To develop UCWA on the Internet for an iOS application, what can I do for the Signin URL when registering the application in Azure AD. Please advise me.
Please refer to the following resource. First you need to register the application in Azure AD, which will provide you with an access token and provide access to Skype for Business. Then, you will actually log into Skype for Business on the Internet.
Set up authentication in Azure Ad and access Skype for Business on the Internet
Sign in to Skype for Business Online