How to Concatenate a string in URL on C#?

Use StringBuilder or for simple cases the + operator.

StringBuilder sb = new StringBuilder()
sb.Append("The start of the string");
sb.Append(theFullNameVariable);
sb.Append("the end of the string");
string fullUrl = sb.ToString();

Or

string fullUrl = "The start" + theFullNameVariable + "the end";

There is a performance penalty to using +, especially if you are using it over several statements instead of one. And in my experiments I've found that about after half a dozen concatenations it is faster to use StringBuilder. YMMV

Use the + operator to concatenate strings. Example:

string html = "asdf" + variable + "asdf";

Remember to use @ on the literal string after the variable also, when you concatenate a variable into a @ delimited string:

string html = @"asdf" + variable + @"asdf";

With your string:

string html = @"<a  style=""width:100%%25;height:100%%25"" href=""http://kcs.kayako.com/visitor/index.php?/LiveChat/Chat/Request/_sessionID=34mh1inqnaeliioe3og5tious2t93ip9/_proactive=0/_filterDepartmentID=/_randomNumber=43/_fullName=" + FullName + @"/_email=usha%40kcspl.co.in/_promptType=chat""  target=""_blank""> <image style=""width:1340px;height:800px"" src=""/Images/1x1-pixel.png"" /> </a>";