Hello, hope you are doing great.
I have been a professional developer since 2014 having experience of 5+ years, I have found the viber API from Nuget gallery (hopefully this library works), I will give it a go, I have read your requirements, regarding your tasks, I Have some questions:
1 - does the messaging include picture and video messages as well ?
2 - Do you have any design in your mind for the User Interface ?
One problem though is that the Viber API Nuget package required minimum .NET Framework 4.6.1, so you have to compromise on that.
Regards,
M. Habib