Custom button in html after field

Code that uses form values and forwards it to a new form/ url:

var url="/wp-admin/admin-ajax.php?action=frm_forms_preview&form=form_key&client=";
var clnt = document.getElementById("field_key");
var uid = document.getElementById("field_key");
var nurl = url+clnt.value+"&uid="+uid.value
function linkWin() {
window.open(nurl, '_self');
}