8 – How to add a button in the header of a table form?

I have this table form:
enter image description here

The corresponding code for the header is

$form('inscriptions') = (
  '#type'   => 'table',
  '#header' => (
    (
      'data'    => '',
      'colspan' => 2,
    ),
    (
      'data'    => 'Distribution',
      'colspan' => 2,
    ),
    (
      'data'    => 'Réserve',
      'colspan' => 2,
    ),
    (
      'data'    => 'Référent',
      'colspan' => 2,
    ),
  ),
  '#id'     => 'planningofdistributions',
  '#sticky' => TRUE,
);

I would like to replace the (empty) content of the first cell by a submit button.
How can I do that?