I found the solution (hurraaay)
first i past the token in a hidden span:
<span id="form-fname">{{lead.First Name}}</span>
then i set the content of the span to a varable in javascript:
var fnavn = $('#form-fname').html();
Then I can target the input field and change the value to my variable:
$('input#FirstName').val(fnavn);
And this also works for checkbox's