you should not connect Bela directly to the S-trigger: the voltage provided by the S-trigger input could cause damage to the board. Instead, you will need a bunch of external components, as per this schematic:
(source and more details: https://www.instructables.com/id/Trigger-Cable-for-Moog-Synthesizer/)
The signal from Bela goes to "Gate Jack Trip" and the input to the S-trig should always be on the collector of the transistor ("Trigger Jack Tip Normal"). When the Bela output goes high, it will directly polarize the base-emitter junction of the 2n3904 transistor, thus allowing current to flow from the collector to ground, effectively replacing a pressed button, as far as the S-trigger input is concerned.