Is this a feature for the backend or frontend?

Backend, Frontend

What would you like?

I'd like to be able to set a custom field when creating a card.

Something like (php):

      $endpoint = "/lists/" . $listId . "/cards";
      $data     = [
          "name"     => $title,
          "position" => 1,
          "type"       => "project",
          "customFields => [
                 "intern" =>
                        [ "SapID" => "5635666" ]
                ]
      ];

Why is this needed?

I want this because I want to automatically create cards from our internal system, but I don't want to make duplicate entries, so I would like to add an SapID to each card, so I can check if the card already exists on the board, before creating a card.

Other information

The API does not seem to provide calls to set a custom field.