As per netsuite documentation for nlapiRequestURL

NetSuite will overwrite the content type with a default type as if the type had not been specified. NetSuite default types are:

  "text/xml; charset=UTF-8"
  "application/x-www-form-urlencoded; charset=UTF-8" 

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I need to  POST to an external URL using MIME content type "application/x-www-form-urlencoded". To accomplish this, I am using the the following code

function fixedEncodeURIComponent(str) {
    return encodeURIComponent(str).replace(/!/g, '%21').replace(/'/g, '%27').replace(/\(/g, '%28').
                                 replace(/\)/g, '%29').replace(/\*/g, '%2A').replace(/%20/g, '+');
}


var POSTData = "tokens=" + fixedEncodeURIComponent(sometokens) + "&order_body=" + fixedEncodeURIComponent(myXML);