I'm sending emails using: https://github.com/sendgrid/sendgrid-nodejs/tree/master/packages/mail
I have not been able to find out HOW I can add the <a href="[Unsubscribe]">Unsubscribe</a> equivalent. This is documented in here: https://sendgrid.com/docs/Classroom/Basics/Marketing_Campaigns/unsubscribe_groups.html#-Using-a-Custom-Unsubscribe-Link
On the website, you just use a shortcode [Unsubscribe], this does not work when sending emails via the sendgrid/mail package.
Since you're sending using code, it's a "transactional" type of message. You'll want to either turn on the Subscription Tracking filter at the account level (via [UI](subscription tracking setting) or API), or turn it on as you send the message, as part of the mail/send API call, under tracking_settings.
It's important to note that you can't mix those. If you define anything in the mail/send API call, you'll need to define everything for Subscription Tracking in that call. SendGrid won't look at some settings at the mail level, and some at the account level.
Most users will just set it at the account level. There, you can customize the HTML & Text of the Unsubscribe footer, customize the HTML of the landing page, or redirect landing to a URL of your choosing, which will send the recipient there with ?email=test@domain.com in the URL string for your system to catch. You can also define the "replacement tag" like [%unsubscribe%], so that you can place the URL wherever you want within your HTML.