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function keypressed(e) {
    if (e.keyCode == 13) {
        e.preventDefault();
        IsValidZipCode(zip.value)
        return false;
        }
    }
        
    function IsValidZipCode(zip) {
        val = document.getElementById("zip").value;
        var isValid = /^[0-9]{5}(?:-[0-9]{4})?$/.test(zip);
        if (isValid) {
            window.open ( "https://client.schwab.com/Public/workshops/resultspopup.aspx?zip="+val);
            document.getElementById("msg").innerHTML = "";
        } else {
            document.getElementById("msg").innerHTML = "PLEASE ENTER A VALID ZIP CODE";
        }
    }

<form id="myform">
  <input id="zip" name="zip" type="text" maxlength="5" placeholder='Zip Code' onkeypress="return keypressed(event)" />
  <button id="go-button" type="button" value="Validate" onclick="IsValidZipCode(this.form.zip.value)" class="submit"></button>
</form>