According to a court document that can be found
here
(https://assets.documentcloud.org/documents/3892131/Trump-Dossier-Suit.pdf)
in a British lawsuit against the author of the Fusion GPS report, Christopher Steele, McCain along with a man named David Kramer were the first people to obtain the Trump dossier.
David Kramer, a former State Department employee who now serves as a senior fellow at the nonprofit McCain Institute, to discuss a meeting he had with Christopher Steele––a former British spy and the author of the controversial dossier––in late November 2016 in London.
Kramer was reportedly directed to meet with Steele in London by McCain, who then received copies of the Trump-Russia dossier and delivered them to the Arizona senator upon returning home. McCain then gave the dossier to the FBI in December 2016.
https://www.politicalreportonline.com/new-evidence-proves-scandalous-collaboration-john-mccain-fbi/