how to generate letsencrypt for web application running on customised port ? for example: erp.example.com:8090 which runs other than 80 and 443
that's not the question. If you have created a certificate, you can use it with every port / application you want.
So the real question: How to create a certificate
- use DNS validation (or)
- use http validation, then an open port 80 / http is required
May be your port 80 application can be used with --webroot.