Connecting the OP-1 Field to a mono guitar pedal. Sum L+R

how do you connect your OP-1 Field to a mono guitar pedal? If you connect a TRS cable to the TS socket on the effects pedal, you only have the left side of the stereo signal, right? Do I have to solder a cable from TRS 3.5mm stereo to TS 6.3mm mono, where both sides are brought together, or can I sum the stereo signal to a mono signal somehow/somewhere in the OP-1?
I’m looking forward to your ideas!

Ok, got it: Sum L+R does not lead to the goal. This video helped me a lot: