Did you try something like this (note: I replaced the XML angled brackets with square brackets so that I can post to this Forum)

[xsl:transform version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"]
  [xsl:template match="/"]
    [out]
      [webRequest]
        [url]http://domain.com/trigger...=value[/url]
        [username][/username]
        [password][/password]
        [domain][/domain]
        [method]POST[/method]
        [postString]{ "SalesOrderNumber": "100", "Name": "John", "Phone": "111-111-1111" }[/postString]
        [timeout]30000[/timeout]
      [/webRequest]
    [/out]
  [/xsl:template]
[/xsl:transform]