How to clear varnish cache using drush? I'm on acquia cloud dev account
With Acquia Cloud API v1 Drush commands

On Acquia Could API v1 the command is ac-domain-purge. Here is a snippet from the drush help available from the Acquia box.

$ drush help ac-domain-purge

Purge a domain from the Varnish cache.

Arguments:
 domain              Domain name. 

Options:
 --username          Acquia Cloud API username                                                                                                                  
 --password          Acquia Cloud API password                                                                                                                  
 --acapi-conf-path   Acquia Cloud API config files location. If not specified config will be loaded from $HOME/.drush                                           
 --caller            Acquia Cloud API caller name. Default is the current username.                                                                             
 --endpoint          Acquia Cloud API endpoint URL.                                                                                                             
 --cainfo            Path to a file containing the SSL certificates needed to verify the ac-api-endpoint.                                                       
 --format            Format to output the object. Use "print_r" for print_r, "export" for var_export, and "json" for JSON. If not provided, the output is printed in a human-readable format.


Additionally, if you want to do this programmatically without drush, Acquia have an API endpoint for clearing the varnish cache.

Note that the drush commands only work with Acquia Cloud API version 1, which is EOL in 2020. 

With Acquia Cloud API v2 requests

Acquia Cloud API v2 is json-api based, and the following API v1 to v2 migration document explains the URL request homologue that replaces each v1 drush command.  In this case:

The V1 drush API command ac-domain-purge becomes one of either:


POST /environments/{environmentId}/domains/actions/clear-varnish

To clear the Varnish cache for all domains used in an environment (eg both the internal appname.prod.acquia-sites.com domain, as well as your site's public domain.
POST /environments/{environmentId}/domains/{domain}/actions/clear-varnish

To clear the Varnish cache for a single specific domain.


See the Acquia Cloud API v2 documentation reference for additional information.