Extending Case Management: how to implement validation in Create Case form?

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The create button to actually create the case is not extendible, so the only other option is field validation. In this case, we need validation on a select-single, which does not support async-validators property. So the final option is to use custom-messages property. We are doing the validation in JS (because we cannot pass virtual fields to action chain..), so my question is how to update the custom-messages from JS, or is there a better way I missed?

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