What should I do for God to forgive my sin after repenting and regretting a previous online relationship, and will this sin be erased from my record, and must I inform my future husband about this relationship if he asks me?

You have done well by regretting the relationship and repenting from it, and repentance atones for sins. Be confident that Allah has forgiven you through your repentance, for He says: "And indeed, I am the All-Forgiving to whoever repents, believes, and does righteousness, and then is guided." You must also conceal yourself and not inform anyone of your sin. We hope that what happened will be a lesson for you not to follow the footsteps of Satan, who commands immorality and wrongdoing.