OpenUrl is a function provided by UIApplication in iOS for calling other programs. In fact, each platform has its own implementation. Here is a cross-platform version directly packaged for everyone.
uses {$IFDEF MACOS}{$IFDEF IOS}FMX.helpers.ios{$ELSE}
sysutils,posix.stdlib{$ENDIF}{$ENDIF}{$IFDEF MSWINDOWS}windows,
shellapi{$ENDIF}{$IFDEF ANDROID}Androidapi.JNI.App,Androidapi.JNI.JavaTypes,
Androidapi.JNI.GraphicsContentViewText, Androidapi.JNI.Net,
Androidapi.helpers{$ENDIF};
procedure OpenUrl(const AUrl: String; const ADataType: String = '');
{$IF DEFINED(ANDROID)}
where
intent: JIntent;
begin
intent := TJIntent.Create;
intent.setAction(TJIntent.JavaClass.ACTION_VIEW);
if Length(ADataType) > 0 then
intent.setDataAndType(TJnet_Uri.JavaClass.parse(StringToJString(AUrl)),
StringToJString(ADataType))
else
intent.SetData(TJnet_Uri.JavaClass.parse(StringToJString(AUrl)));
intent.setFlags(TJIntent.JavaClass.FLAG_ACTIVITY_NEW_TASK);
TAndroidHelper.Activity.startActivity (intent);
{$ELSEIF DEFINED(MSWINDOWS)}
begin
ShellExecute(0, nil, PChar(AUrl), nil, nil, SW_SHOWNORMAL);
{$ELSE}
begin
{$IFDEF IOS}
SharedApplication.OpenUrl(StrToNSUrl(AUrl));
{$ELSE}
_system(MarshaledAString(TEncoding.Default.GetBytes('open ' + AUrl)));
{$ENDIF}
{$ENDIF}
end;