You can't - the table name has to be available at SQL "compile time" and can't be a variable.
There are ways around this: you can build a NVARCHAR command in SQL and use EXEC to execute it, or you can concatenate strings to produce the SQL command you send from your C# code.
But...and it's a big "but"...both are dangerous and have to be done with extreme caution. Concatenating strings is dangerous - it exposes your code to SQL Injection, so you have to be very, very aware of exactly what
can get assembled into the string that gets sent to SQL if you want to keep your database intact! If your users can provide the text at all, you are at a very real risk of damage or deletion if they (for example) enter
cars; DROP TABLE cars;--
and you concatenate that into your string or NVARCHAR value.
Personally? I wouldn't do it: I'd use fixed strings and select the appropriate one from a list instead of building them in any way.