As per spec a client can only send a HEADERS frame to cause a stream to
transition from the "idle" state to "open". A PRIORITY frame can be sent to an
"idle" stream, but it will remain in that state.

rfc7540,l,916,940
https://tools.ietf.org/html/rfc7540#section-5.1

To open a stream the command

txreq -nostrend

can be used. The -nostrend option will ensure that the stream won't transition
to a "half-closed" state.