How to add/suppress custom scope in AWS Pre-Token Generation trigger ?

I have Cognito user pool with one Allowed custom scopes for my app client i.e. admin-only. I have two kinds of users 1. Users in Admin Group 2. Non-Admins users. For my one of the AWS API Gateway Routes, I need to deny the access if user from non admin group is hitting the API Url, basically its allowed only for users which are part of Admin User group.

I can achieve it through adding an Authorization scope on API gateway route with this custom scope and then adding a scope manually when I request an Authorization token in Hosted UI popup. But in my app, I don't want to add this scope manually, rather want to add this scope when the token is generated. I explored Pre-Token Generation Trigger but not able to see the way to override or add scope attribute in it. Also tried below code in Pre-Token Generation Trigger lambda.

exports.handler = async (event, context, callback) => {
event.response = {
        "claimsOverrideDetails": {
            "claimsToSuppress": ["admin-only"]
        }
    };
    callback(null, event);    
};

Question is, is there a way to add/override custom scope in pre-token generation trigger ? If yes then how ? But if there is no way, then how to solve my use case ?