As of today, for Snowflake on AWS target endpoint there is no option to provide proxy settings in Advanced tab when Storage type is selected as Snowflake.
Today in order to include proxy you need to manipulate the json.
NOTE: Upon clicking this link 2 tabs may open - please feel free to close the one with a login page. If you only see 1 tab with the login page, please try clicking this link first: Authenticate me! then try the link above again. Ensure pop-up blocker is off.