Mine is a DC Blue Plus. Probably over 10 years old. The initial plan was to power it from the DC Blue's 12v output, but after thinking about it I was a bit worried about 12V causing damage to the main board over time, so I am using a 5V 500mA power supply from an old phone.
The Sonoff 1 chan wifi switch is wired up to the Common output (C) and N/O Button trigger input (BT). Add the device in your eWeLink app, join your LAN, then setup the eWeLink skill in Alexa. Tell Alexa to scan for new devices, then add a routine to open garage and close garage. Tell Alexa 'Open Garage' -> Win!