[PHP, React] Distributed Tracing

It is not very clear from the documentation how to combine several transactions under a single TraceID.

I use the standard BrowserTracing integration from the documentation.

Problem 1:
Documentation on Javascript SDK says:

If your frontend is making requests to a different domain, you will need to add it there to propagate the sentry-trace header to the backend services, which is required to link transactions together as part of a single trace.

This works perfectly by sending the sentry-trace header in queries for the domain api.example.com.
Backend and frontend transactions of the main page are linked via the meta tag.
However, XHR for the current domain do not include this header. How do I combine main page and XHR transactions in this case?

Problem 2:
How do I bind XHR transactions to the initial TraceID? Will it be enough to use SpanContext::fromTraceparent when initializing the context of the first Span?
$spanCtx = \Sentry\Tracing\SpanContext::fromTraceparent($_SERVER['HTTP_SENTRY_TRACE']??'');