AWS - Cannot read "domainName" of undefined - amazon-web-services

Question

[EDIT]
I am new to AWS and I've been trying to make a real-time chat application. I saw this code, so I might as well try this. I tried to follow everything but it seems there is a problem.
[END]
I got an error with this code stating, Cannot read "domainName" of undefined which got me confused. Is this because of my API Gateway? I already connected this to another route named "Message" and I already have the integration request of my connect and disconnect.
Below is my code:
const AWS = require('aws-sdk');
const db = new AWS.DynamoDB.DocumentClient({region: 'us-east-1'});
require('./index.js');
let send = undefined;
function init(event) {
console.log(event);
const apigwManagementApi = new AWS.ApiGatewayManagementApi({
apiVersion: '2018-11-29',
endpoint: event.requestContext.domainName + '/' + event.requestContext.stage
});
send = async (connectionId, data) => {
await apigwManagementApi.postToConnection({
ConnectionId: connectionId,
Data: `Echo: ${data}` }).promise();
}}
exports.handler = (event, context, callback) => {
init(event);
let message = JSON.parse(event.body).message;
getConnections().then((data) => {
console.log(data.Items);
callback(null, {
statusCode: 200,
});
data.Items.forEach(function(connection) {
console.log("Connection " +connection.connection_id);
send(connection.connection_id, message);
});
});
return {};
};
function getConnections(){
const params = {
TableName: 'GroupChat',
}
return db.scan(params).promise();
}