Laravel 5 Merge an array

I am trying to merge an array with an input from a form of a random string of numbers

In my form I have

<input type="text" name="purchase_order_number" id="purchase_order_number" value="{{ $purchase_order_number }}" />

And in the controller:

public function store(CandidateRequest $request)
{
    $candidateInput = Input::get('candidates');
    $purchaseOrderNumber = Input::get('purchase_order_number');

    foreach ($candidateInput as $candidate)
    {

        $data = array_merge($candidate, [$purchaseOrderNumber]);

        $candidate = Candidate::create($data);

        dd($data);

When I dd($data) it’s getting my purchase_order_number but as seen below but how can I assign it to that row in the table?

array:6 [▼
  "candidate_number" => "5645"
  "givennames" => "fgfgf"
  "familyname" => "dfgfg"
  "dob" => "01/01/2015"
  "uln" => "45565445"
  0 => "5874587"
]

Many thanks,

via Chebli Mohamed