There is a really very nice feature hidden in here:
https://powerapps.microsoft.com/en-us/tutorials/field-groups/#lookup-and-app-generation-with-field-groups
Unfortunately it does not work yet.
Here is how to reproduce the bug:
1) Create two custom entities, one Countries one Cities
2) In Cities create a relationship to lookup to Countries and call it Country
3) In Countries add a text field and call it "MyTextValue"
4) In Countries, edit the DefaultLookup field group (as decribed in the link above) and add both PrimaryId and MyTextValue
5) Create a PowerApp and connect to Cities
6) Create a text box and start typing its text value as First(Cities).Country. and then you should get a choice of PrimaryId and MyTextValue
So far so good, and it would be great if it worked. Unfortunately if you choose myTextValue the text box does not show the corresponding value of MyTextValue for the first record in Cities, but shows a blank.
