Changed Related ID to correctly use Campaign ID

Salesforce Send Mass Email from Campaign Members (uses built-in email).js

////////////////////////////////////////////////////////////////
// Title: Salesforce Send Mass Email from Campaign Members (uses built-in email)
// Description:
// Allows for mass Salesforce emails without templates, with simply a click.
// Note that you could easily go over Salesforce's limits if you're not careful.
//
// By Alex Kadis, based on sources listed below, especially 
// Matt Horine's "Email Affiliated Contacts button for use with
// Salesforce NPSP"
// 
// Requires Salesforce NPSP
// 
// Directions:
// 	Setup > Create > Objects > Affiliation > New Button or Link
// 		Display Type: List Button
// 		Behavior: Execute JavaScript
// 		Content Source: OnClick JavaScript
// 		Paste in code below
// 	Setup > Accounts > Page Layout > Edit your page layout
// 	Scroll down to Affiliated Contacts > Properties (wrench icon)
//	> Buttons > Add your newly available button
// 
// Sources:
// https://developer.salesforce.com/forums/?id=906F00000008rTJIAY
// https://success.salesforce.com/answers?id=90630000000h3K8AAI
// https://developer.salesforce.com/forums/?id=906F00000009BDSIA2
// http://501partners.com/salesforce-tip-email-many-contacts-at-once/
// https://gist.github.com/mdhorine/929c3fc48ef62b566b0e
//
// Similarly useful: create new contact with certain details
// https://developer.salesforce.com/forums/?id=906F00000008jRMIAY
////////////////////////////////////////////////////////////////

{!RequireScript("/soap/ajax/20.0/connection.js")}

var contactRecords= {!GETRECORDIDS($ObjectType.CampaignMember)};


var csvContactIds = "";
//fetch csv format Ids
for(var rowNum in contactRecords){
    csvContactIds += "'"+ contactRecords[rowNum] + "',";
}

//remove last comma
csvContactIds = csvContactIds.slice(0, csvContactIds.length - 1);

//Enclose the ids in round brackets
if(csvContactIds.length > 1){
    csvContactIds = "(" + csvContactIds + ")";
}

// Email parameters:
 
// template_id = id of email template you want to pre-populate
// email_type=visualforce (if the template that you're using is a VF email template)
// new_template=1   = required if you're using a VF email template that contains an attachment
// p2_lkid = id of the record for the To address (a Contact or Lead)
// rttype = the id prefix for the type of the To address (e.g., if p2_lkid refers to a Contact, use rttype=003) See: https://help.salesforce.com/articleView?id=Standard-Field-Record-ID-Prefix-Decoder&language=en_US&type=1
// p3_lkid = id of the record for the Related To (i.e., What) record (e.g., an Account or Opportunity)
// p4 = CC address
// p5 =  BCC address
// p6 = Email subject
// p7 = Email body
// p24 = Additional To
// p26 = From address
// doc_id = the id of a Document or QuoteDocument to become an email attachment
// retURL = URL to return to after the email is sent
// cancelURL = URL to return to if the user clicks Cancel

var defaultTemplateID = " ";
var toId = " "; // cannot be a user
var toIdPrefix = "003";
var relatedID = "{!Campaign.Id}";
var ccEmails = " "; 
var additionalTo = "{!$User.Email} ";
var contactEmails = " ";

// return to current URL
var retURL = encodeURIComponent(window.location.href);


if (contactRecords.length ==0) {
    alert("Please select at least one Campaign Member record.");
} 
else {
	
    var result = sforce.connection.query('SELECT Contact.Name, Contact.Id, Contact.Email FROM CampaignMember WHERE Id IN ' +csvContactIds); 
    var records = result.getArray("records"); 
    
    var email = "";
    for (var i=0; i< records.length; i++) {
        email += records[i].Contact.Email + '; '; 
    }

    contactEmails += email;

    window.location.href= "/email/author/emailauthor.jsp?"

    +"&template_id="+defaultTemplateID
    +"&p2_lkid="+toId
    +"&rtype="+toIdPrefix
    +"&p3_lkid="+relatedID
    +"&p4="+ccEmails
    +"&p5="+contactEmails
    +"&p24="+additionalTo
    +"&retURL="+retURL
    +"&cancelURL="+retURL;
 
}




// create new contact with certain details
// https://developer.salesforce.com/forums/?id=906F00000008jRMIAY


// Email affiliated contacts
// https://gist.github.com/mdhorine/929c3fc48ef62b566b0e