Where can I find the join and replay links?

We pass the join and replay links to all autoresponders as custom fields.  

Below is a picture of Active Campaign but every autoresponder gets the same custom fields:

28 prefix is the Webinar ID

Webinar Date - the date they registered for the webinar

Webinar Time - the time they registered for the webinar 

Time Zone - The registrant time zone 

Live Room Join Link - the link to join the live webinar

Replay Room Join Link  - the link to join the replay

How would I use this in an automation/email?

Every autoresponder providers custom field variables/personalization tags that you can use in your emails.  In Active Campaign they look like this but you can reach out to your specific autoresponder to figure out what they are: