To create a custom account field, the following permissions are required.

Account permission: the user should have permission to manage account.

{	
	"accountCustomFieldMetum": {
		"fieldLabel": "New Title",
		"fieldDefault": "Default Text",
		"isFormVisible": 1,
		"displayOrder": 1
	}
}

{
    "accountCustomFieldMetum": {
        "id": "1",
        "fieldLabel": "New Title",
        "fieldType": "text",
        "fieldOptions": null,
        "fieldDefault": "Default Text",
        "isFormVisible": 1,
        "isRequired": 1,
        "displayOrder": 1,
        "createdTimestamp": "2018-10-22 19:57:37",
        "updatedTimestamp": "2018-10-22 20:04:21",
        "links": {
            "accountCustomFieldData": "https://:account.api-us1.com/api/:version/accountCustomFieldMeta/1/accountCustomFieldData"
        }
    }
}