How do I store the json value as string in C#

In C#, you can do it in two ways:
1) Prefix the string with an atsign, which disables the escape character "\" and then double up each double quote:

string s = @"[{""Id"":""908F08bc-024E-4371-9F1b-a15156f9fe85"",""It"":false,""Ch"":[]}]";

Or
2) Use the escape character before each double quote:

string s = "[{\"Id\":\"908F08bc-024E-4371-9F1b-a15156f9fe85\",\"It\":false,\"Ch\":[]}]";

But the code you show doesn't match the tags: that is VB code, which doesn't use a backslash escape code - so you have to double up double quotes:

Dim s As String = "[{""Id"":""908F08bc-024E-4371-9F1b-a15156f9fe85"",""It"":false,""Ch"":[]}]"

Decide which language you are using and always tag it appropriately: different languages need different solutions!

Quote:

tại sao chúng tôi không nhìn thấy nó rõ hơn
(Why can't we see it more clearly?)

Because the string has to have delimiters: the double quotes indicate to the system where it starts and stops. So if you don't either have an escape character or double the delimiter up when you want one inside the string it assumes the first double quote it meets is the end of the string - it can't "read your mind" and work out what you intended to do!

The alternative is as Richard said: use Raw String Literals (if you are using .NET 7 & C# 11.0 or above). In that case you can write this:

string s = """
           [
             {
             "Id":"908f08bc-024e-4371-9f1b-a15156f9fe85",
             "It":false,
             "Ch":[]
             }
           ]
           """;

Which is even more readable than the single line version.

string s = """[{"Id":"908f08bc-024e-4371-9f1b-a15156f9fe85","It":false,"Ch":[]}]""";

string s = $$"""[{"Id":"{{Guid.NewGuid():N}}","It":false,"Ch":[]}]""";

There is a simple solution to this problem that was introduced in C#11 ( see the comment from Graeme_Grant for further details). Use two string interpolation characters $ to escape the formatting characters { and }, followed by three double quotation marks used to escape the quotation marks that are in the text. Then place your text on a new line followed by another newline containing three double quotation marks and a semi colon.

string test = $$"""
[{"Id":"908f08bc-024e-4371-9f1b-a15156f9fe85","It":false,"Ch":[]}]
""";
Console.WriteLine(test);