Can't tag prefab with a string

Hi,
I’m trying to add a simple string tag to my prefabs:
if file name suffix is “_NC”, i want to add the tag “Choose”.
I looked online and so that all I have to do is prefab.tag,
So I wrote the following code:
if (fileName.Substring(snackName.Length - 3).Equals(“_NC”))
prefab.tag = “Choose”;

But I get the error:
UnityException: Tag: Choose is not defined.

How can I fix it?
I saw in some places that perhaps tags can be created only by editor.
If it is indeed the case, how do I create a tag and use it in the code, when I am creating some new prefabs?

Thanks

Edit → Project Settings → Tags and Layers.

I believe that’s what you’re looking for!