how to start HTTPS for an application running in an AWS beanstalk environment?

Hello.

If you have successfully created an SSL certificate with ACM, you can link the certificate to the load balancer using the steps described in the document below.
Issuing SSL certificates with ACM requires issuing them on your own domain.
Please note that ACM cannot issue certificates for the default domain assigned to ElasticBeanstalk (elasticbeanstalk.com).
https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/configuring-https-elb.html

By the way, NLB is a layer 4 load balancer, so it cannot create an HTTPS listener.
https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/environments-cfg-nlb.html

Unlike a Classic Load Balancer or an Application Load Balancer, a Network Load Balancer can't have application layer (layer 7) HTTP or HTTPS listeners. It only supports transport layer (layer 4) TCP listeners. HTTP and HTTPS traffic can be routed to your environment over TCP. To establish secure HTTPS connections between web clients and your environment, install a self-signed certificate on the environment's instances, and configure the instances to listen on the appropriate port (typically 443) and terminate HTTPS connections. The configuration varies per platform. See Configuring HTTPS Termination at the instance for instructions. Then configure your Network Load Balancer to add a listener that maps to a process listening on the appropriate port.