Hi!
it´s an exact copy from this blogpost:

$prod_data = [
	'name'        => 'A great product',
	'type'        => 'variable',
	'description' => 'A very meaningful product description',
	'images'      => [
		[
			'src'      => 'https://pathTo.jpg',
			'position' => 0,
		],
	],
	'categories'  => [
		[
			'id' => 1,
		],
	],
	'attributes'  => [
		[
			'id'        => 5,
			'variation' => true,
			'visible'   => true,
			'options'   => [ 'S', 'M', 'L' ],
		],
	],
];

$this->product = $this->api->post( 'products', $prod_data );

		$variation_data = [
		'regular_price' => '35.00',
		'image'         => [
			'src' => 'https://pathTo.jpg',
		],
		'attributes'    => [
			[
				'id'     => 5,
				'position' => 0,
				'option' => 'S',
			],
		],
	];
	
	$id = $this->product->id;
	
	$this->api->post( "products/$id", $variation_data );