Hello hubijo,
You can change the 12th bit (D11) in register 2078 (hex 081E) first by selecting the HW Configuration window and the 170x\140x Register Control tab, which brings up the view shown below. Then click Master Mode in the Serial Output section. This puts a 1 in D11 of register 2078, and switches the I2S output to master mode. As a result, BCLK becomes active.
Or, if you prefer you can change the desired bit directly by typing it into the register box on the same screen:
Best regards,
Bob