Can you please provide some details on how to do it?
What I did isn't necessarily the most efficient, engineering-wise (if I had to do it from scratch, I'd do it differently), but it is fast and inexpensive...
1) Buy a TP-Link Kasa wireless AC switch or similar. These interface directly with Alexa without the need for a hub, currently $14 on Amazon:
https://www.amazon.com/TP-Link-Occupies-HS105-Wall-Light-Electronic-Component-switches/dp/B01K1JVZOE
2) Buy a simply 5V relay, 5V DC coil, with relay that is suitable for 120VAC, low current. It could be a module with connectors like this ($6.00):
https://www.amazon.com/Tolako-Arduino-Indicator-Channel-Official/dp/B00VRUAHLE
or even just a raw 5VDC relay like this:
https://www.amazon.com/QWORK-SRD-05VDC-SL-C-5Pins-Power-Relay/dp/B085ZX7LVJ
3) Find or buy a simple 120VAC to 5VDC charger wallwart, like any old USB charger. You probably have several lying around, or get one at the Dollar store, or this (overkill): it just needs to provide 120VAC->5VDC at 100ma...
https://www.amazon.com/Android-Adapter-2000mah-Charger-Matricom/dp/B073VYWBGK
4) Optional, a project box, like this:
https://www.amazon.com/Awclub-Waterproof-Dustproof-Universal-100mmx68mmx50mm/dp/B07CG2B9H3
The idea is simple:
- Alexa tells the TPLink Kasa wireless switch to turn ON (120VAC)
- The 120VAC powers the USB charger
- The 5VDC turns ON the relay
- The relay acts like the garage door opener button.
Alexa >> TPLink >> 5VDC charger >> 5V relay >> garage opener switch terminals closed
Wire the NO (normally open) relay output (switch) across the opener terminals where the opener switch goes -- in parallel, the existing switch can stay. This project took me maybe 20mins to make and fit in a small project box -- another 5mins to wiring into the opener setup. Total cost < $30.
The last tidbit is that a standard opener switch is "momentary", a push button, whereas the TPLink is ON or OFF. So go to the Alexa App and create a simple "routine", called Garage or similar. The routine is simply Turn ON Kasa switch, wait 5secs, Turn OFF Kasa switch. Then you just say "Alexa Garage" to activate the routine.
The major "con" of this method, is that there is no knowledge of if the door is already open or not -- the same is true for the standard garage opener switch. In practice, it hasn't been a problem. But if you want to do better, just put a wireless sensor (like a window security sensor) on the garage door that Alexa can read and have the Routine account for the state of the garage door. I haven't bothered with this though...
Let me know if you have more questions....