Resolved: Input user name and email id and download pdf certificate with same name (name+emailid) from a folder

     function myFunction()
    {
        var downloadUrl = document.getElementById("url").value;
        document.getElementById("downlod").href = '/ooecertificate' + downloadUrl + '.pdf';
    }
    
    alert("You are Successfully Called the JavaScript function");  
     }

<div>
        <label>ENTER YOUR REGISTRATION ID:</label> 
        <br> </br>
        
        <label>(Your registration id is your firstname+registered email id:
                for example if your name is SAMMY DIXIT and 
                your registered email id is SAMMY.DIXIT@GMAIL.COM
                your REGISTRATION ID WILL BE: SAMMYSAMMY.DIXIT@GMAIL.COM
        </label> 
        
        <input type="text" name="url" id="url" onkeyup="myFunction()"> 
        
        <a href="/OOEcertificate" id="downlod" download><span>Download</span></a>
        
    </div>
    </div>

function downloadPdf(){    
    
    //fetch the registrationId from the input#url
    let registrationId = document.getElementById("url").value;        
    
    //transform the registrationId string to lower case
    registrationId = registrationId.trim().toLowerCase();
    
    const pdfUrl = `/ooecertificate/${registrationId}.pdf`;        
    
    //this is the statement to redirect the browser to the wanted url
    //now commented because it wouldn't work here on the snippet
    
    //window.location.href = pdfUrl;
    
    //on its behalf I'm writing the url on console
    console.log(pdfUrl);
}

label.fieldLabel{
  display: block;
  margin-bottom: 1rem;
}

label.fieldDescription{
  display: block;
  margin-bottom: 1rem;   
  border: solid 1px gray;
  padding: 1em;
}

button{
  cursor: pointer;
}

<div>
  <label class="fieldDescription">
  Your registration id is your firstname+registered email id:<br>
  For example..<br>
  if your name is SAMMY DIXIT and your registered email id is SAMMY.DIXIT@GMAIL.COM<br>
  your REGISTRATION ID WILL BE: <b>sammysammy.dixit@gmail.com</b>
  </label> 

  <label class="fieldLabel">ENTER YOUR REGISTRATION ID:</label>   

  <input type="text" name="url" id="url"> 

  <button onclick="downloadPdf();">Download</button>
  
</div>