How can one seek forgiveness for a deceased parent, and should it be with the intention on behalf of the deceased, or in a specific format?

Seeking forgiveness (Istighfar) means asking for forgiveness, and saying "Astaghfirullah" means asking God for forgiveness. Seeking forgiveness for a deceased parent is permissible and commanded, and it is a form of showing righteousness towards them after their death. There is no specific formula for seeking forgiveness; rather, it is sufficient to supplicate for mercy and forgiveness for them with any wording that conveys this meaning, such as saying "O Allah, have mercy on him" or "O Allah, forgive him."