If user is uninstalling your application, it is reasonable to ask him what he did not liked in the application. Usually developers open web browser for submission of feedback.
IMHO this is not a good practise — most users do not like applications opening web browser without their confirmation. I consider it is a lot better to implement sending of such kind of feedback using desktop's regular mail application, such as Outlook and Thunderbird.
For example, this is IndieVolume Uninstall Feedback window:

And, here is InnoSetup pascal code I have developed to implement above window:
function UninstallFeedback(): Boolean;
var
Form: TSetupForm;
FeedbackLabel0: TLabel;
FeedbackLabel: TLabel;
OKButton, CancelButton: TButton;
FeedbackMemo: TMemo;
Feedback: String;
Url: String;
ErrorCode: Integer;
begin
Result := false;
Form := CreateCustomForm();
try
SetSystemFont(Form.Font);
Form.ClientWidth := ScaleX(350);
Form.ClientHeight := ScaleY(10+23 +10+46 +10+23 +10+23+10 + 23);
Form.Caption := 'IndieVolume Uninstall Feedback';
Form.BorderIcons := [biSystemMenu];
Form.BorderStyle := bsDialog;
Form.Center;
OKButton := TButton.Create(Form);
OKButton.Parent := Form;
OKButton.Width := ScaleX(75);
OKButton.Height := ScaleY(23);
OKButton.Left := Form.ClientWidth - ScaleX(75 + 6 + 75 + 20);
OKButton.Top := Form.ClientHeight - ScaleY(23 + 10);
OKButton.Caption := 'Send';
OKButton.ModalResult := mrOk;
OKButton.Default := true;
CancelButton := TButton.Create(Form);
CancelButton.Parent := Form;
CancelButton.Width := ScaleX(75);
CancelButton.Height := ScaleY(23);
CancelButton.Left := Form.ClientWidth - ScaleX(75 + 20);
CancelButton.Top := Form.ClientHeight - ScaleY(23 + 10);
CancelButton.Caption := 'Cancel';
CancelButton.ModalResult := mrCancel;
CancelButton.Cancel := True;
FeedbackMemo := TMemo.Create(Form);
FeedbackMemo.Parent := Form;
FeedbackMemo.Width := Form.ClientWidth - ScaleX(30 + 20);
FeedbackMemo.Height := ScaleY(61);
FeedbackMemo.Left := ScaleX(30);
FeedbackMemo.Top := Form.ClientHeight - ScaleY(23 + 10 + 10) - FeedbackMemo.Height;
FeedbackLabel0 := TLabel.Create(Form);
FeedbackLabel0.Parent := Form;
FeedbackLabel0.AutoSize := False;
FeedbackLabel0.Left := ScaleX(30);
FeedbackLabel0.Top := ScaleY(10);
FeedbackLabel0.Width := Form.ClientWidth - ScaleX(30 + 20);
FeedbackLabel0.Height := ScaleY(42);
FeedbackLabel0.WordWrap := True;
FeedbackLabel0.Caption := 'We regret you have uninstalled IndieVolume. ' +
'To help us with future versions of IndieVolume, ' +
'we want to know about any troubles or difficulties ' +
'you have experienced while using IndieVolume.';
FeedbackLabel := TLabel.Create(Form);
FeedbackLabel.Parent := Form;
FeedbackLabel.Left := ScaleX(30);
FeedbackLabel.Top := ScaleY(10+42+10);
FeedbackLabel.AutoSize := True;
FeedbackLabel.Height := ScaleY(21);
FeedbackLabel.Caption := 'Please let us know...';
FeedbackLabel.FocusControl := FeedbackMemo;
Form.ActiveControl := FeedbackMemo;
if Form.ShowModal() = mrOk then
begin
Feedback := Trim(FeedbackMemo.Lines.Text);
if Feedback <> '' then
begin
StringChange(Feedback, '&', '%26');
StringChange(Feedback, #13#10, '%0A');
StringChange(Feedback, #13, '%0A');
StringChange(Feedback, #10, '%0D');
Url := 'mailto:support@gerixsoft.com?Subject=' +
'IndieVolume%20Uninstall%20Feedback&Body=' + Feedback;
ShellExec('open', Url, '', '', SW_SHOW, ewNoWait, ErrorCode);
end;
end;
finally
Form.Free();
end;
end;
procedure CurUninstallStepChanged(CurUninstallStep: TUninstallStep);
var
WindowNumber: HWND;
var
RegDir: String;
begin
if CurUninstallStep = usUninstall then
begin
UninstallFeedback();
end;
end;