If I understand what you want to do, I think the nested Datagrid should be contain Artikel objects instead of BestelopdrachtRegel objects. Once you’ve made that change, you can create a small microflow with two parameters: Artikel and BestelopdrachtRegel. In this microflow, you’ll need a change Activity that changes BestelopdrachtRegel and sets the association with the passed in Artikel object. Finally, place an Action button in the control bar of the datagrid that points to this microflow.
After doing that, when you select a row in the datagrid and click the button, the association will be set.
You could also check out this widget, which provides the ability to set the value of a reference with built in searching: https://appstore.home.mendix.com/link/app/2695/AuraQ-Limited/AutoComplete-widget
Hope that helps,