@F_Quint
If this is a ComboBox, then you will want to set the DefaultSelectedItems property not the Default.
In your DefaultSelectedItems, place the following formula (assuming that your _myProfile has an email address as well):
With({profile:Office365Users.MyProfile()},
{Claims: "i:0#.f|membership|" & profile.Mail,
Department: profile.Department,
DisplayName: profile.DisplayName,
Email: profile.Mail,
JobTitle: profile.JobTitle,
Picture: "."
}
)
Make sure you have AllowSearching on.
I hope this is helpful for you.