Whenever you want to add/update/delete an additional permission, just send back the updated list of permissions, through this PUT request to the API. For example if you initially had two additional permissions e.g {additional_permissions: {api_developer: API Developer, api_manager: API Manager}} to add a third permission called custom_permission you would append it to the original permission list and your request body would be: {additional_permissions: {api_developer: API Developer, api_manager: API Manager, custom_permission: Custom Permission}}.
The Tyk Dashboard API Access Credentials
{
"api_developer": "API Developer",
"api_manager": "API Manager",
"custom_permission": "Custom Permission"
}