Interacting selectively and non-covalently with a specific DNA sequence in an RNA polymerase II (Pol II) core promoter, the region composed of the transcription start site and binding sites for transcription factors of the Pol II basal transcription machinery, in order to promote assembly of the transcriptional preinitiation complex (PIC), the formation of which is a prerequisite for transcription from an RNA polymerase II promoter.