Is the hadith about a man seeking the Prophet's help at his grave during the time of Omar, when the land was afflicted by drought, authentic and can it be used as evidence for the permissibility of seeking the Prophet's help?

The Hadith of Al-Utbi is fabricated and false. What is mentioned in it is tawassul (seeking a means) not istighathah (seeking aid). Istighathah is an act of worship that cannot be directed to anyone other than Allah in matters that none but Allah can accomplish. Tawassul includes what is permissible, such as seeking a means through the supplication of a righteous man, as Umar did with Al-Abbas, and what is forbidden, such as seeking a means through one's status.