我已经弄明白了。您只需将标题和块（不是对象）添加到读取/自定义块。因此......

I've figured it out. You just add 'header and a block (not an object) to the read/custom block. Thus...

http-custom-header: [
    Content-Type: text/xml
    etc...
]

xml-request: {
    <?xml version="1.0" encoding="utf-8"?>
    <etc>etc...<etc>
}

site-URL: http://etc...

response: read/custom site-URL reduce [
    'header http-custom-header
    'post xml-request
]