You need the base url of your shopware shop, root url, domain of the sales channel – yes that is for SEO, so I find next time my own post to copy paste the solution:
{{ salesChannel.domains.first.url }}Just in case you don’t know what happens: salesChannel is imho always passed to the mails. We take the sales channel, get the domains, take the first() (of the collection) and getUrl.
Or in other words:
$data['salesChannel']->getDomains()->first()->getUrl()