How to set custom schema for custom remote methods on Strongloop

I believe you might have gone through the official docs of strongloop. If not, here is the link that explains the remote methods and their accepted data types. https://docs.strongloop.com/display/public/LB/Remote+methods

Assuming your custom object is Challenge, to show the object in response you have to specify the type( the type can be one of the loopback's data type or you custom model). So to return Challenge you have to add following code :

Challenge.remoteMethod('score', {
    accepts: { arg: 'data', type: 'object', http: { source: 'body' } },
    returns: {arg: 'scores', type: 'Challenge', root: true},
    http: {path: '/score', verb: 'post'}, 
});

The second arrow that you have specified is the default values that you want to try out with your API call. You can pass any custom string with default as the key. For example, if you want to pass some object :

Challenge.remoteMethod('score', {
    accepts: {
        arg: 'data',
        type: 'object',
        default: '{
            "id": "string",
            "userId": "string",
            "user": {},
            "totalScore": 0,
            "tags": []
        }',
        http: {
            source: 'body'
        }
    },
    returns: {
        arg: 'scores',
        type: 'Challenge'
    },
    http: {
        path: '/score',
        verb: 'post'
    }
});

So, for response you can not customize the model. But to pass default values you can put anything in the string format.

In loopback, remote arguments can identify data models which have been defined using ds.define('YourCustomModelName', dataFormat);

so for you case, write a function in a Challenge.js file which will have a remote method (in ur case score) defined.

const loopback = require('loopback');
const ds = loopback.createDataSource('memory'); 
module.exports = function(Challenge) {
  defineChallengeArgFormat() ;
 // remote methods (score) defined
};

let defineChallengeArgFormat = function() {
  let dataFormat = {
            "id": String,
            "userId": String,
            "user": {},
            "totalScore": Number,
            "tags": []
        };
  ds.define('YourCustomModelName', dataFormat);
};

Under remote arguments type use 'type': 'YourCustomModelName'

    Challenge.remoteMethod('score', {
        accepts: {
            arg: 'data',
            type: 'YourCustomModelName',
            http: {
                source: 'body'
            }
        },
        returns: {
            arg: 'scores',
            type: 'Challenge'
        },
        http: {
            path: '/score',
            verb: 'post'
        }
    });

You should see it reflecting on explorer after restarting server and refreshing :)