Insert value into field using Custom Script

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Hi All,

I have a requirement for a report that calculates a value based on some pre-populated fields in the database and then inserts that value into a custom field by way of Custom Script. I have the report constructed to calculate the value already but I am struggling to figure out how to insert it into a field via an API call. I've checked a few documents as well as searching here and have not found the answer to my query (or have not been able to figure it out from the info on hand).

For info, the value itself will be available in a hidden column, as will the ID of the Incident I need to update, and these are pulled into variables in the script. The bit I'm missing is updating a custom field with the calculated value for the Incident ID in question.