How to add field values to UDF field (type: picklist) using REST API

How to add field values to UDF field (type: picklist) using REST API

Hi,
I have a scenario where, let's say we have users logging request for computer equipment(eg laptop mouse, etc.) but the request must have a valid project number.  The valid list of project numbers exist in an external system(eg Oracle Financials). I have created a UDF field (type: picklist) for the project number on the incident template but I want the available list of values for the UDF field to updated each night using REST API. How can I achieve this?
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