...
"metadata": {
    "event_type": "",
    "event_payload": null
},
...

package handlers

import (
	"github.com/slack-go/slack"
)

func main {
	api = slack.New("xoxb-xxx")

	// Define PostMessageParameters
	messageParameters := slack.PostMessageParameters{
		MetaData: slack.SlackMetadata{
			EventType: "reaction_added",
			EventPayload: map[string]interface{}{
				"event_id": "123",
			},
		},
	}

	// Post a message to a channel with the created messageParameters
	_, _, err := api.PostMessage(
		"channel-name",
		slack.MsgOptionText("Hello", false),
		slack.MsgOptionPostMessageParameters(messageParameters),
	)
}