Did you look up an exiting one ?
They must have a 'story_use' action and the SignPost.itemScript item script assigned. It's probably enough to enter a newly created 'translated string key' in the SignPost parameters when assigning the script.
Create your own new string key in the 'Translated String Key' editor. It's the icon between the 'Keyword editor' and the 'Texture Atlas Editor'.
I have never tested sing posts myself, but I assume, this is how it works. Looked up the sign outside the Cyseal north gate.
Region names are also translated string keys. If you create a new region, the editor will complain about not finding a name for the new region. You can enter a region name the exact same way, it does not even matter how you name the file or the files if you create more than one.
The string key files you create in the editor will end up in Data\Mods\<YourMod>\Localization. The game picks them up from there.
All messages you display - for example in Osiris with CharacterDisplayText() - are translated string keys. If you create a new stat entry for a new item, you either enter the name in the root template directly or you enter the item's name as a translated string key with the name of the stat as the key. If neither the root template nor the stat that uses it have a translated string key, there will be an empty name field in the tooltip for the item.
So it is pretty important to know this tool. (Kind of a key-value database.)
If you extracted the .pak files, you can find the used keys in Data\Mods\Main\Localization\Signposts.lsb. If you unpack this one - there's probably an .lsf version in MainLSF.pak - you can see it in clear text in an XML file. (The signpost I looked up uses "CYS_North2Sign" which can be found there.)
Note that all predefined string keys have a pointer into the localization file (that's the handle="..." field withing the entries, which you can only see in unpacked LSBs). The editors 'TranslatedStringKey' editor will not give you a chance to enter this pointer in your own string key file. ... and we don't have a simple way to provide localization anyway, so it does not matter.
