function send() {

    var inputs = document.getElementById('frm1').getElementsByTagName('input');

    var str = '';

    for (var i = 0; i < inputs.length; i++) {

        str += inputs[i].name + '=' + inputs[i].value + '%0a';

    }

    window.location.href = 'mailto:mailaddrss@server.co.il?body=' + str;

}

<form action="mailto:mailaddress@server.co.il" method="post" enctype="text/plain">

    FirstName:<input type="text" name="FirstName">

    Email:<input type="text" name="Email">

    <input type="submit" name="submit" value="Submit">

</form>

<form id='frm1'>

    FirstName:<input type="text" name="FirstName">

    Email:<input type="text" name="Email">

    <input type='button' name='btn' value='Submit' onclick='send()' />

</form>