gistfile1.txt

array(
	'name' => __( 'Date and time', 'theme' ),
	'slug' => 'date_time',
	'description' => __( 'Date and time of the event.', 'theme' ),
	'type' => 'date_v2',
	'type_date_v2_options' => array(
		'show_as' => 'datetime',
		'show' => 'on_click',
		'default_date' => 'today'
	)
)

// Field extensions store their options in the options array, grouped by field key
// It should (and will be in future) added by just using options directly, but for now this is the way it works

array(
    'name' => __( 'Date and time', 'theme' ),
    'slug' => 'date_time',
    'description' => __( 'Date and time of the event.', 'theme' ),
    'type' => 'date_v2',
    "options" => array(
        "date_v2" => array(
            "show" => "on_click",
            "show_as" => "datetime",
            "default_date" => "today"
        )
    )
)

Worked like a charm. Thanks!