Painless - New Calculated field as a .keyword extension

In our data we have a page field with URLs and some come in as /blog/really-good-content/ and others come as /blog/really-good-content (the lack of trailing slash)

So in Painless I've made a scipted field to merge these records so I can analyze them together. This is fine. My issue is I don't know how to then use the .keyword extension, required for TSVB tables, against this new field. Even if I make calculated field just
doc['Page.keyword'].value (none of the fancy substring stuff) I can't use it in tsvb charts.

What am I missing, how do I use Painless to make a scripted calculated field that will have a .keyword extension that I can then use in a TSVB?

Thank you

Scripted fields are currently not supported in TSVB, there's an issue for that in our git repository, you could use to track or comment there

Maybe for your use case you could use runtime fields

With runtime fields you can add a painless script in the mapping of you index