@Anonymous
What are you storing in the single line of text column in your SharePoint list?
If it is the Email address of the person (which it should be), then set the DefaultSelectedItems property of your Combobox in your app to the following:
{
Claims:"i:0#.f|membership|" & Lower(Parent.Default),
Department:"",
DisplayName:Parent.Default,
Email:Parent.Default,
JobTitle:".",
Picture:"."
}
I hope this is helpful for you.