Peace be upon you. According to Ayatollah Khamenei, it is not sufficient to simply dedicate the reward of good deeds to the person; one must seek forgiveness from them if they are still alive and it is possible to do so. If the person is deceased or if seeking forgiveness is not possible, then one should ask for forgiveness on their behalf. Ayatollah Sistani advises that it is recommended to seek forgiveness from the person backbitten or to ask for forgiveness for them. If seeking forgiveness is considered a way to rectify the harm caused by backbiting, it is obligatory to do so, provided it does not lead to further harm. It is not necessary to know the specific details of what the backbiting was about unless there is evidence that the forgiveness does not cover such backbiting. May you be successful in all good deeds.