Try access_url (for certain pages also website_url)

UPDATE

For sure, access_url does not exist in vacuum, it is an attribute of the model. I guess, you mean in your question the model 'helpdesk.ticket'. The object itself might be referenced differently depending on used controller. If I’m not mistaken in portal templates it should be ‘ticket’, in email templates it is object.

Besides, what do you mean by href="${access_url}"?? Dynamic variables should be used with ‘t-’ positioning. E.g.:

 <a t-attf-href="${object.access_url}">link</a>

 Take also in account that email templates and portal pages have different syntax. So, for example, ‘$’ is hardly applicable to sites.

i tried: <a href="${access_url}">link</a> and <a href="${website_url}">link</a> but that did not work...