No it was not. It was part of the evidence presented to the FISA judge by the FBI to obtain a warrant to monitor Carter Pages communications AFTER he left the campaign, and the FBI had been interested in Page before the Trump campaign even existed.[1]
The investigation into Trump's campaign and its Russian contacts was NOT started by the dossier. The FBI was not given the dossier until December of 2016. By this time there was no campaign anymore since Trump was president-elect. In fact the investigation had already started months prior when Papadopolous made suspicious comments to a diplomat about the campaign's knowledge of information related to Hillary's emails. [2]
Ah, it seems McCain gave the dossier he received to the FBI in December but the "FBI confidential human source" had already provided the information to them.
But October 2016 is still after Page left the Trump campaign, and after the Russian investigation had started.
