Shortcut Dimension 3 and above is not actual field on the table.
You need to use variable and trigger to update the dimension set id .
You can follow from General Journal Page on how to do that.
Below is the sample code :
var
ShortcutDimCode: array[8] of Code[20];
field(ShortcutDimCode3; ShortcutDimCode[3])
{
ApplicationArea = All;
CaptionClass = '1,2,3';
TableRelation = "Dimension Value".Code WHERE("Global Dimension No." = CONST(3),
"Dimension Value Type" = CONST(Standard),
Blocked = CONST(false));
trigger OnValidate()
begin
ValidateShortcutDimCode(3, ShortcutDimCode[3]);
end;
}