How to post on a Facebook group as a Page, using the Graph API Explorer?

Question

I am the administrator of website, a Facebook group, a Page and a Facebook API. I would like to create, once a day, a post on my Facebook group using my Page name as author AND using the Graph API Explorer. Is it even possible?

To start simply, I use the Graph API Explorer directly but it fails to create the post. https://developers.facebook.com/tools/explorer/

For the moment, I have tested the Graph Explorer with these data and parameters:

Request: post/v15.0/{my_group_id}/feed?message=test

Settings:

• Application Meta: My app
• User or Page: My Page
• Permissions: pages_show_list, publish_to_groups, pages_read_engagement, pages_manage_posts, public_profile

{
  "error": {
    "message": "Permissions error",
    "type": "OAuthException",
    "code": 200,
    "error_subcode": 1376025,
    "is_transient": false,
    "error_user_title" => "No Permission to Post"
    "error_user_msg" => "You do not have permission to post in this group."
    "fbtrace_id": "xxx"
  }
}

I'm not sure if I'm doing something wrong or if it's just impossible?