[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Registry]
Root: HKLM; Subkey: "Software\My Company\My Program\Settings"; ValueType: string; ValueName: "IPAddress"; ValueData: "{code:GetIPAddress}"

[Code]
var
  IPAddressEdit: TNewEdit;

function GetIPAddress(Value: string): string;
begin
  Result := IPAddressEdit.Text;
end;

procedure InitializeWizard;
begin
  IPAddressEdit := TNewEdit.Create(CustomPage);
  ...
end;