per @Kevin-Reid's answer, here's an alternative to the "I ended up doing the following" example that avoids needing to name and then lookup the form object again by constructing the form specifically (using jQuery)..
var url = 'http://example.com/vote/' + Username;
var form = $('<form action="' + url + '" method="post">' +
'<input type="text" name="api_url" value="' + Return_URL + '" />' +
'</form>');
$('body').append(form);
form.submit();
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