C# strings use double quotes as a start/end markers, so inside a string there are two ways to include them: escape them with a backslash prefix
"\""
or use a verbatim string literal by prefixing the whole string with an
@
character and then using two double quotes together:
string a = "Hello \"World\"";
string b = @"Hello ""World""";
In your case because you want to embed backslash characters as well, the verbatim string format is probably best as is completely disables escape characters:
string s = @"string contain another string(< a href =\""https:"")";
string text = @"This is a \""string\"" in C#.";
string text1 = @"This is a \na test \nb.";
Printing these gives the result you want:
string contain another string(< a href =\"https:")
This is a \"string\" in C#.
This is a \na test \nb.
Although I suspect you meant to use
string s = @"string contain another string(<a href=""https:\\"")";