Is it obligatory for a couple who committed fahishah (an immoral act) before their marriage to build a mosque as expiation for their sin, and will Allah forgive them for what they did?

Committing the indecency of fornication is among the major sins, and its perpetrator must hasten to sincere repentance, which entails immediately abandoning the sin, regretting what has passed, and resolving never to return to it. If repentance is achieved, then Allah Almighty forgives all sins, as He said: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" It is recommended to increase good deeds such as prayer, fasting, and charity, so that Allah may expiate that sin through them, "Indeed, good deeds do away with evil deeds." As for building a mosque as expiation for this indecency, it is not correct, unless it is intended as a general increase in good deeds.