An option would be to use variables and the variable webhooks.
For a particular device (let's call it switch_1) create a new variable called switch_1_status with a value of on or off.
Now, create a new sequence that looks something like the following. In this example I'm using it to control an eWeLink device named Christmas tree but you could also use a Tuya or Philips Hue device as well.
You can import this sequence directly into your account from here.
Now, you should head back to the variables page and click the little plug icon next to switch_1_status. This will take you to the variables webhook page where you can retrieve a webhook to either toggle the value, or specifically set it to on or off.
Something to be aware of is that if you also control this device outside of SEQUEmatic the value of switch_1_status may not stay in sync with your actual device. You might want to change the trigger to check if the variable is modified rather than changed. It's a subtle difference, but changed only triggers if the value changes - ie from on to off or off to on. Modified will trigger if it's modified but doesn't actually change - eg on to on or off to off.
Hopefully that helps, but if you have any questions don't hesitate to ask!