<Location /api>
    RequestHeader set MY_HEADER "value"

    ProxyPass http://127.0.0.1:8000/api
    ProxypassReverse http://127.0.0.1:8000/api
</Location>

RequestHeader set X-My-Header "value" expr="req('X-My-Header')==''"

SetEnvIf X-My-Header "" no_my_header
RequestHeader set X-My-Header "value" env=no_my_header