You might find the two phrases confusing. So, here are a brief explanation about the two phrases and sample sentences.
In nontechnical sense, ‘Log on’ means visiting (without using a username and password) while ‘log in’ means signing in with a username and password (or- sign in).
Sample sentences:
The user can log on to a website operated by such entity to read the message.
Facebook now requires Belgium users to log in to view pages.