[SalesForce] OnClick JavaScript and Commandbutton

I have a Visualforce page with a custom button that calls JavaScript to execute a script. When I click the button, I get a 404 Error (Not Found) error. Apparently, this is due to an SFDC resource called "logo180.png". It seems this is a known issue that occurs by design to prevent malware. However, it prevents by button from working on my VF page. Does anyone know how I can avoid this?

<apex:page standardController="Engage_Short_Form__c">

<script type="text/javascript" src="/apex/dsfs__DocuSign_JavaScript"></script>
<script type="text/javascript">
    function sendViaDocuSign() {
        var CRL = ''; 
        var DST = ''; 
        var CES = '';
        CRL = 'Email~{!JSENCODE(Engage_Short_Form__c.Partner_Email__c)};FirstName~{!JSENCODE(Engage_Short_Form__c.Partner_Contact_First_Name__c)};LastName~{!JSENCODE(Engage_Short_Form__c.Partner_Contact_Last_Name__c)};Role~Signer1,Email~jneilan@outbrain.com;FirstName~John;LastName~Neilan;Role~Signer 2'; 
        DST = '0E9AB63C-62C0-4D01-B91A-E58849B7B573'; 
        CES = 'Insertion Order signature request for {!JSENCODE(Engage_Short_Form__c.Partner_Legal_Name__c)}';
        window.location.href = "/apex/dsfs__DocuSign_CreateEnvelope?DSEID=0&STB=1&SourceID={!Engage_Short_Form__c.Id}&LA=0&CRL="+CRL+"&DST="+DST+"&CES="+CES; 
    }
</script>



   <apex:form >

            <center><button class="btn" onclick="sendViaDocuSign()">Send via DocuSign</button></center>