How to create a custom welcome email template like this
Make sure you've configured the registration widget to activate an account after a confirmation has been sent to the user. To do this check the following documentation article: https://www.progress.com/documentation/sitefinity-cms/account-activation-widget-webforms
The content of the confirmation email is taken from a label resource which can be modified from Sitefinity's backend by navigating to Administration > Labels & Messages and searching for the RegistrationConfirmationEmail label. The template contains few values such as <%\s*ConfirmationUrl\s*%> which are replaced dynamically before the email is sent to the user. You can edit the resource label so that it best fits your case.