The tracking pixel looks like this:
<img width="1" height="1" style="display:none" alt="" src="https://t.sidekickopen04.com/s3t/o/5/f18dQhb0S7n28bp_z8W5jHQkK2zGCvGW40Fv_600000VzJsdVcGjW4XyJ_-56dQb2dsM00z02?si=8000000020100194&pi=d5578900-7ad5-465d-932d-cd3787790000">
I believe its generated via the Hubspot Sales extension. Haven’t found a way to generate a pixel myself yet.
There’s a bit more info here: Understand HubSpot Sales email open and click tracking | Knowledge Base