Accessing Web Vital TBT on a Private Instance

I have recently set up a private instance and have it working well and have found CLS and LCP in the JSON output.
However, what is necessary to get TBT available as well.
I can see it on UI-based requests but seem unable to surface the metrics through the API (for the same URL).
Any insight on this would be appreciated.

Pass timeline=1 as a test option. Timeline capture is necessary to capture the main-thread activity. It is enabled by default in the UI usually but needs to be requested through the API.

Thanks Pat, applied this and it works fine now.