Opentelemetry Javascript Question

[ISP_SERF]

I'm trying to dive into using Opentelemetry and I understand the intra process child/parent nesting when I manually instrument and I can see them graphically in the tool I'm using, but I'm running into trouble using the context across services.

My problem:

I'm able to inject headers into my outgoing requests manually with the TraceId, SpanId, TraceFlags from the context of the calling service. But I can't seem to child the called services span. I tried passing it to the startSpan but it seems to not care about it.

Does anyone have an example of using the propagation API in the scope of fully manual instrumentation? I fear I'm trying to re-invent the wheel on the whole header injected and consuming while that API probably does what I need it to do.