var form = document.createElement("form");
form.setAttribute("method", "post");
form.setAttribute("action", xxx);

function submitToPopup(f) {
    var w = window.open('', 'form-target', 'width=600, height=400, any-other-option, ...');
    f.target = 'form-target';
    f.submit();
};

document.body.appendChild(form);

submitToPopup(form);