Get the WhatsApp user name who visits my website Page using ASP.Net C#

I have created a question page on my website using ASP.Net,C# and SQL Server. Its Url is like "www.mywebsite/questions.aspx". On this page, there are some questions with 4 options. If a user gives the answer of the questions then i am saving the question and his answers. i want to share my website page link "www.mywebsite/questions.aspx" on Whatsapp. Now, I want to save the Whatsapp user name who is giving the answer of the question with question and answer into database using ASP.Net,C# and SQL Server. How i will do it, please suggest me.