You must enrol for the class 2B course with an authorised driving centre and attend the theoretical lessons before you can book for the Riding Theory Test (RTT) through the authorised driving centres as stated:

Bukit Batok Driving Centre (BBDC)
815 Bukit Batok West Ave 5
Singapore 659085
Tel : 6561 1233

ComfortDelGro Driving Centre Pte Ltd
205 Ubi Ave 4
Singapore 408805
Tel : 6848 0617

Singapore Safety Driving Centre (SSDC)
2 Woodlands Industrial Park E4
Singapore 757387
Tel : 6482 6060

This information is sourced from SPF-STAGING