During this era, apps weren't downloaded from a centralized "App Store." Instead, users downloaded (Cabinet files)—the Windows Mobile equivalent of an Android .apk or a Windows .exe. You would transfer the file to your device via microSD or ActiveSync and run it to install the software. The Search for WhatsApp Messenger .cab
Popular HTC models with Windows Mobile 6.1: whatsapp messenger.cab for htc windows mobile 6.1
Do not download from random “warez” sites. Windows Mobile 6.1 has no modern antivirus, so a malicious .cab can brick your device. During this era, apps weren't downloaded from a