@Jasper1234
You are describing a one to many relationship between your lists. First of all, the relationship is defined by a primary key in the Client list and a foreign key in the Orders list. Ideally, this should be the ID from the client list. I would assume at some point you are choosing a client from either a gallery or another control such as a dropdown or combobox that is bound to the Client list. I Assuming a dropdown, it would need to be outside the form itself and its Items property would be Sort(Client, ClientName, Ascending). Although the dropdown shows only one record in the list, all of the fields in the list can be referenced. If your client number is unique and can not be reused if you remove the client from the list, I would use that since clients may have the same name, otherwise, I would recommend using the ID column of the Client list. This is a hidden number that SharePoint automatically assigns but it can be referenced in the app. In your Orders list, I would have a column called clientID that links the two lists.
The scenario you are describing implies a new Order. If you want to be able to edit existing orders, you will need to add another control that depends on which client is selected to show the existing orders.
The form should have the Datasource property Orders and the Item property, Lookup(Orders, clientID = Dropdown1.Selected.ID). In the form, there will be a card containing a textinput box control for clientID. Set the Default property to Dropdown1.Selected.ID and then either hide the card or set its display property to disabled. This way, the user cannot change this value.
Addendum: I just realized that you are using Excel and not SharePoint. Excel does not automatically assign an ID to a table. In that case you can use your client number instead. However, it should not be reused If deleted or it will cause problems in the future.
