Forbidden

I’ve tried to send new message reply with signature via API:
POST https://api2.frontapp.com/conversations/cnv_xyz/messages

 However I’ve got an error like this:

{
    "_error": {
        "status": 403,
        "title": "Forbidden",
        "message": "This agent is not allowed to read the \"signature\" with ID: \"1234567\""
    }
}

So I tried to list signatures of my account like this:

GET https://api2.frontapp.com/teammates/tea_xyz/signatures/
But  I’ve got something similar

{
    "_error": {
        "status": 403,
        "title": "Forbidden",
        "message": "You do not have access to the resources of this teammate."
    }
}

What should I do in order to send message with signature?
Where should I add some rights/permission?