Hi,
I am building a Dynamics 365 Custom Page.
I load the Order Lines Table from Dataverse and save it in a variable:
App.OnStart:
Set(OrderLinesData; Filter('Order Lines'; Order.'Order ID' = orderRecord.'Order ID'));;
AddColumns(OrderLinesData; "SelectedQuantity"; 0)
Additionally I am trying to add a "temporary column" "SelectedQuantity" which is initialized with a Value of 0.
I then display the OrderLinesData in a Gallery using the items property (works fine):

I can successfully show all default Dataverse Columns, but I struggle displaying the value of my temporary column that I created:

It cannot resolve the SelectedQuantity attribute in the gallery.
It also does not work at Runtime.
I believe I must be misusing the AddColumns function or I am missing something related to how variables/collections work.
I don't want to update any real value in the source Dataverse table, but just want to be able to freely work with the collection I am loading from Dataverse.
Any ideas? Thanks!