Oh man, adding module to Vtiger top menu not super hard, but kinda confusing if you're new. So, first, you make the module, yeah? Like in /modules/ folder. Add all files there, PHP stuff. After that, go to database, table vtiger_tab. Add a new row for your module name. Make sure it has isentitytype=1, so Vtiger knows it's legit.

Then for menu, there's a table called vtiger_parenttab. You add your module to a category there, or it won't show in the top menu. If still invisible, clear cache or log out/in. About WhatsApp pop-up, you'll need API, like Twilio. Pop-ups? Use Vtiger’s JS modal things. Might take time, but works if you're patient.