For all of you struggling after flashing a Sonoff Basic with Tasmota firmware and then wanting to toggle the relay like I did:
In the WHEN column: add a virtual button or other activity that can occur to trigger the flow. Leave the AND column empty for now. In the THEN column add the Homey logic card ‘Make a web request’.
On that card change the Method from GET to POST, in the next field (URL) type HTTP://SonoffIPaddress/cm?
Fill the field HEADER with the text: “Content-Type:application/HTTP” without the ".
In the BODY field type the text: “cmnd=Power%20Toggle” without the ".
Please like this post if you too were helped by this.