You need to set up an IFTTT applet that is compatible with Siri and for that it must be available to Shortcuts. To begin with that you need to make sure it has a trigger that is a widget type service trigger.
The services available are Button widget, Note widget and Camera widget. Each has a trigger and available as a widget to add to the home screen. Button is a simple control trigger, note presents a text field box to enter a note when triggered and camera runs your camera app to allow you to take a photo as part of the trigger. Since we want to use it ultimately with Siri and control by voice we have to use the Button widget only.
1. Go to the Create page in the IFTTT app.
2. Under If This select Add.
3. Choose or search for Button Widget.
4. Then add any actions, queries or filters you want to this Applet.
You have your Applet ready so now you can create a Shortcut on your iPhone using it.
To create the Shortcut, open the Shortcuts app.
1. Select the "+" on the Shortcuts homescreen.
2. Select, "Add action".
3. Navigate to "Apps" and tap the IFTTT icon.
4. Select "Trigger Applet".
5. Select the Applet you want to trigger from Shortcut compatible list.
6. Name your Shortcut. This will be the custom phrase you will use to trigger your Applet.
With this you can set up any kind of Shortcut you want with all the usual Shortcut triggers, knowing it will also trigger IFTTT actions.
By having a Shortcut set up it's now very easy to control via Siri. Simply use the name or custom phrase you set for the Shortcut to refer to it via Siri.