How should checkbox behave if it has a link inside text field? Is it a good UX if only underlined link links to another page when the rest of the text in the control and the box itself check the checkbox?
I would open a new tab / modal rather than letting the user change page just to read the rules.
What if the user has already filled out everything and then is redirected to another page?
Sure you could store what the user typed; but I think it would cause frustration being redirected.
Remove the link and add it to the end of the line, away from the checkbox label:
[ ] I accept rules of the website. (Click here to read rules)
Two things have been acheived: 1- You kept the checkbox mental model (the whole link checks/unchecks the choice) 2- You clearly stated "click here" which removes any confusion between the checkbox label functionality and the link (don't let me think).