Explanation:

Following is the command

aws cloudwatch set-alarm-state --alarm-name <Created alarm name> --state-reason <comment> --state-value ALARM

Explanation:

Launch an EC2 instance for which the custom CloudWatch memory metrics need to be generated.

Create an IAM role named "CustomMetricsRole" with a custom policy name "CustomEC2CloudWatch" to establish the interoperability between EC2 instance and CloudWatch as below
Policy : CustomEC2CloudWatch
Permissions:
{
"Version": "2012-10-17",
   "Statement": [
     {
      "Sid": "VisualEditor0",
      "Effect": "Allow",
      "Action": [

          "cloudwatch:GetMetricStatistics",
          "cloudwatch:PutMetricData",
          "cloudwatch:GetMetricData",
          "ec2:DescribeTags",
          "cloudwatch:ListMetrics"

       ],
       "Resource": "*"
     }
  ]
}
Attach IAM role to the EC2 instance

SSH to EC2 instance
$ sudo apt update
$ sudo apt install unzip
$ sudo apt install perl
$ sudo apt install liblwp-protocol-https-perl libdatetime-perl

Download the CloudWatch Monitoring perl scripts for Linux
$ wget http://aws-cloudwatch.s3.amazonaws.com/downloads/CloudWatchMonitoringScripts-1.2.1.zip
$ unzip CloudWatchMonitoringScripts-1.2.1.zip
$ cd aws-scripts-mon

mon-put-instance-data.pl - Collects system metrics on an EC2 instance and sends them to CloudWatch
mon-get-instance-stats.pl - Queries CloudWatch and displays the most resent metrics
AwsSignatureV4.pm - File template for AWS credentials

Push custom metrics from EC2 instance to CloudWatch
$ ./mon-put-instance-data.pl --mem-util --mem-used --mem-avail

Create crontab and add the script to run at every minute intervals.
$ crontab -e
* * * * * /home/ubuntu/aws-scripts-mon/mon-put-instance-data.pl --mem-util --mem-used --mem-avail

To retrive the most recent custom metrics from CloudWatch to EC2 instance
$ ./mon-get-instance-stats.pl