How to edit the sysconfig default value for the requestable field on "New Entitlement"

Which IIQ version are you inquiring about?

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Under “Application → Entitlement Catalog → Add new entitlement”, I want to change the “requestable to be selected by default. However due to this is a sailpoint code, I will need to figure a way to override option in sysconfig.”

You need to write Group Aggregation Refresh Rule

check the sample code below