Adding text to beginning of text file

I am trying to add a line of text to the beginning of a text file (Actually not text, but just two backslashes).
I am trying the following, which I found in this answer: Add lines to the beginning and end of the huge file

$ sed -i '1i\'"$\\" $Simdata.txt

However, I cannot seem to get it to function.
When I use the above, I get the error:

sed: 1: ".txt": invalid command code .

I have tried without the txt ending, but then I get this error:

sed: -i may not be used with stdin

I also tried this line of code, which I found somewhere else:

$ sed -i '1i \\' Simdata.txt
sed: 1: "Simdata.txt": invalid command code S

UPDATE:

@don_crissti, your first solution is what I want to do. However, it does not seem to function as intended.

$ cat Simdata.txt
abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk
$ sed '1i\
> \\\\' Simdata.txt
\\abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk
$ cat Simdata.txt
abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk

So the file is not updated. I would prefer not to print the file, just update the file or print to a new one. Also, I made a silly typo: I would actually like to add two forward slashes.
What I am ultimately aiming at doing is the following.

I have a file looking like this:

abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk

I want to first add // to the beginning of the file:

//
abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk
//
abcdefghijkabcdefghijk

And then add a unique name after each //:

// text 1
abcdefghijkabcdefghijk
// text 2
abcdefghijkabcdefghijk
// text 3
abcdefghijkabcdefghijk

and save this to a new file, without changing the original file. So the above question was aiming at figuring out the first step.