Added a line below. PUT to the draft (/invoices/drafts/:id) and it'll be updated. Remember to PUT the whole document with the additional/changed data as PUT overwrites existing.
'lines'=> [
[
"lineNumber"=> 2,
"product" => ["productNumber"=> "500"],
"quantity"=> 1.00,
"unitNetPrice"=> 200,
"productName"=> 'Ur oprettelse'
],
[
"lineNumber"=> 3,
"product" => ["productNumber"=> "500"],
"quantity"=> 2.00,
"unitNetPrice"=> 200,
"productName"=> 'Another line added'
]
],