In the ODK Collect v1.16 Beta thread, @Fabla brought up that it would be useful not just to add SMS as a way to send forms but also to send messages from the form to do things like thank participants.
I believed this was already possible using the external app feature but it turns out that's not true. While that feature allows us to launch different applications and to pass special values called Android extras to them, the
android.intent.action.SENDTO message requires knowing who to send the messages to and that's not done through extras.
I think this would be very useful to add. See this sample form for how I imagine it being used (doesn't work in Collect today).
The narrowest way I can think of supporting this in Collect is to introduce a special key in the parameter list that allows the form designer to specify a recipient(s). I would like to propose
uri_data as its name since that is unlikely to already be part of forms and because it matches what it will represent from an Android perspective (see
data in https://developer.android.com/reference/android/content/Intent).
I have a pull request up at https://github.com/opendatakit/collect/pull/2351 for any devs who might have ideas.