https://tasmota.github.io/docs/devices/Sonoff‑T1/ 
Configuration > Configure Module > Sonoff T1 3CH
immediately after flashing Tasmota (so it won't reset when powered from adapter):
SetOption13 1

mapping: https://github.com/arendst/Tasmota/blob/a466c5abf33386c3dda34696e11ae975c7ac6649/tasmota/tasmota_template.h#L2468 
 {                     // SONOFF_T13 - Sonoff T1 3CH (ESP8285)
    GPI8_KEY1,          // GPIO00 Button 1
    GPI8_USER,          // GPIO01 Serial RXD and Optional sensor
    GPI8_USER,          // GPIO02 Optional Sensor (J3 Pin 5)
    GPI8_USER,          // GPIO03 Serial TXD and Optional sensor
    GPI8_REL3,          // GPIO04 Blue Led and Relay 3 (0 = Off, 1 = On)
    GPI8_REL2,          // GPIO05 Blue Led and Relay 2 (0 = Off, 1 = On)
                        // GPIO06 (SD_CLK   Flash)
                        // GPIO07 (SD_DATA0 Flash QIO/DIO/DOUT)
                        // GPIO08 (SD_DATA1 Flash QIO/DIO/DOUT)
    GPI8_KEY2,          // GPIO09 Button 2
    GPI8_KEY3,          // GPIO10 Button 3
                        // GPIO11 (SD_CMD   Flash)
    GPI8_REL1,          // GPIO12 Blue Led and Relay 1 (0 = Off, 1 = On)
    GPI8_LED1_INV,      // GPIO13 Blue Led (0 = On, 1 = Off) - Link and Power status
    0, 0, 0, 0
  },