Here is what I did (I have tried to include all the steps - apologies if it is unnecessarily detailed or if I have omitted something)....
Go to Devices, Add device, select Virtual, give the device a name and type and number of physical buttons (I chose "Virtual Doorbell Button" and Virtual Button and 1). Make a note of the device ID number (one way of doing this is by looking at the address bar of your browser - the last number in the address will be the device ID)
Go to Apps, Add built-in app, select and install Maker API. Open Maker API and under "Allow Endpoint to Control These Devices" select your virtual device. Under Local URLS, Send Device Command, copy the http.... command.
In your Synology, Edit Action Rule, Action, paste the http... command. Replace [Device ID] with the device ID number of your virtual button, replace [Command] with push, replace [Secondary value] with 1.
That's it. Do a test send. In my case I used the button push events to trigger actions in Rule Machine.....
