That was likely how they got warrants, by not telling the FISA court that the info was fake.
Via Daily Caller:
Both the House and the Senate are investigating whether the former President Barack Obama’s administration used intelligence in a salacious “Trump Dossier” as “evidence” before a secret federal surveillance court to obtain permission to spy on Donald Trump campaign aides and later his transition team.
Leaders in Congress are trying to find out if the Obama Justice Department and the FBI introduced the totally unsubstantiated material alleging unsavory connections between Trump and Russia before the Federal Intelligence Surveillance Court, congressional aides told The Daily Caller News Foundation.
The FBI used the dossier to secure permission to monitor the communications of Trump associate Carter Page, based on U.S. officials briefed on the Russia investigation, CNN reported in April.