checkbox_basic.json

{
    "type": "checkboxes",
    "action_id": "and...action",
    "options": [
        {
            "text": {
                "type": "plain_text",
                "text": "A"
            },
            "value": "A"
        },
        {
            "text": {
                "type": "plain_text",
                "text": "B"
            },
            "value": "B"
        }
    ],
    "initial_options": [
        {
            "text": {
                "type": "plain_text",
                "text": "A"
            },
            "value": "A"
        }
    ]
}