Capitalisation likely matters. Field probably needs to be named RefersTo. :hammer:
The field name likely isn't RefersTo. On the HTML form, it's new-RefersTo :sob:
I'll need to look at the RT source code to look at if this field is even supported by the REST API. I have a sneaking suspicions it's only supported in the 4.x series of RT. :astonished:
I'm unsure if the 'refersto' (or indeed, the other fields under links in RT) is intended to be supported, but it currently does not work: