I hope the steps below help you get a double opt in.. There are 2 choices available at this time.. The Below was written out for use in Mailchimp but can be used in any autoresponder, you only need to use that autoresponders shortcodes/merge fields.. As seen below.
1: Create a custom registration page on say your main site or any page builder that can collect the name and email into your MailChimp account - mailchimp form or MC integration for this page
2: then create an autoresponse to go out inside MC for after they (confirm) double opt in and are added to the list.. in this auto response email you will use the easywebinar 1 click url modified for mailchimp.. this url is found in publishing options and is the last url..
3: modify this 1 click url by replacing the capitolized NAME and EMAIL in that url with the mailchimp shortcodes or your autoresponders merge fields that will fill in first name and email which would make the end of that url look like this ( attendee_name=*|FNAME|*&attendee_email=*|EMAIL|* ) -- THIS VIDEO MAY HELP https://youtu.be/Cgu9riRfNGY
4: make that newly modified url into a hyperlink in that email behind text or an image so when they click on it it will auto register them into that easwebinar event and land on the EW thank you page and trigger the welcome email inside EW
5: set up your notifications inside EW event for welcome which is immediate the before ( as many as you like, 2-5 roughly ) and NO NEED to turn on autoresponder as they are already in your mailchimp
This will ensure they are truely double opted in (confirmed) and will not get EW notifications until they click on that hyperlinked 1 click url in that first email sent from mailchimp after confirming.. This also means you dont have to fill in any blanks on the registration page tab in EW as you are using a custom page with your mailchimp form on it..
but what if I want to give them options for date and time in my recurring event?
yes this is true the 1 click url will put them into the 1st available slot so now this can make you stuck between ensuring you get double opt ins b4 the ew details or the selection of times.. As it stands right now those are your choices because mailchimp does not give you an option for making them single opt ins like getresponse or aweber and others..
The only other way to ensure you get a double opt in is to not use EW built in notifications at all then create a form to add into EW with name, email and a custom field for event url to get that registrants unique url into your MC, also within this form (general forms) set up your thank you pages like this http://screencast.com/t/LKct04YDV but I also want you to watch this https://youtu.be/774P4ocj978 because you can redirect to EW thank you page after opt in as well as send out 1 email response from MC after they opt in to get their unique url.. In that first response out will have that merge field event url for Mailchimp to autofill that registrants unique url..