To place the Parley bet you need to build the combination on the betslip prior sliding to place the bet.
After you select your first bet, slide down the betslip and continue navigating the offers by the arrow on the top left. Once the second selection is added you will see the Double on top and the menu 1xCOMBINATION SINGLES SYSTEM.
For the Parley stay on COMBINATION tab, just put down the betslip for navigation and keep on adding the selections. Once you have added all bets, slide to bet.