multi_select_static.json

{
    "type": "multi_static_select",
    "action_id": "multi_static_select",
    "options": [
        {
            "text": {
                "type": "plain_text",
                "text": "A"
            },
            "value": "A"
        },
        {
            "text": {
                "type": "plain_text",
                "text": "B"
            },
            "value": "B"
        }
    ],
    "placeholder": {
        "type": "plain_text",
        "text": "Select one or more"
    }
}