If you mean, a keyword added to the export - see this previous thread: https://forums.adobe.com/thread/1719085
If you mean, a keyword added to the Catalog metadata for that image - then I would probably look at this the other way around. Maybe give the image a particular keyword first, and then have a Published Smart Folder carry out the export - where possessing this keyword is a criterion for membership in that Smart Folder. Also, the image could show in a normal Smart Collection according to that keyword.
Alternatively: http://regex.info/blog/lightroom-goodies/collection-publisher may be of interest here.
Thanks for the tips
How do I create a Published Smart Folder?
Smart Folder is an option within a Publish service. This can be set up to export images to local storage, or else to some online destination such as Flickr. Lightroom uses slightly different naming depending on what kind of a service is involved, but when publishing to disk, it calls it a "folder" because that is what the destination consists of.
And it calls it a Smart Folder when you employ stated rules - along the same lines as when you make a Smart Collection instead of a Collection - which dynamically change the membership depending on properties of your images. Then when you tell it to, it carries out the export process for the relevant images.
Caution: this operates on the theory that you want the destination folder to reflect whatever is set in the Catalog at a given moment. So if you have applied a keyword which caused an image within the Catalog to be included in a Publish Smart Folder, and accordingly had a copy of that image made into the destination folder when you clicked Publish last time - and now you remove this keyword from the image concerned: this removes it from the Publish Smart Folder: so when you next click Publish, the copy image previously made, will accordingly be deleted from the destination folder. Just so you know .
If you envisage this use of a special keyword as being just a momentary thing, and not an enduring attribute of these images - then you may be better off setting up a Smart Collection which refers to this keyword along with perhaps other criteria, and then Exporting the contents of that as and when appropriate.