Hi,
You should call those events for your table in the codeunit or you can write the below code On OnInsert trigger and OnModify Trigger as well.
[EventSubscriber(ObjectType::Table, database::"YourTable", 'OnAfterInsertEvent', '', true, true)]
local procedure OnAfterInsertEvent_YourTable(var Rec: Record "YourTable")
begin
if (rec."Customer No." <> '') and (rec.code <> '') then
rec."External Document No." := rec."Customer No." + ' ' + rec.Code;
end;
[EventSubscriber(ObjectType::Table, database::"YourTable", 'OnAfterModifyEvent', '', true, true)]
local procedure OnAfterModifyEvent_YourTable(var Rec: Record "YourTable")
begin
if (rec."Customer No." <> '') and (rec.code <> '') then
rec."External Document No." := rec."Customer No." + ' ' + rec.Code;
end;