content type – Update field default value on a subfolder REST


I have multifolder structure like that:

Opportunities (document library of custom type OPFolder)
     OP0026 (folder of custom type OPFolder)
        Incoming (normal folder)

custom type OPFolder has field OPNumber

I wanted to update Default value for this field at last level e.g. Incoming folder
I’ve tried to post $"{{ '__metadata': {{ 'type':'SP.Field' }}, 'DefaultValue': 'Hello' }}"; to

https://{site}/_api/web/Lists/GetByTitle('Opportunities')/RootFolder/Folders('OP0026')/Folders('Incoming')/ListItemAllFields/OPNumber

Request succeeded but nothing gets set.

Note: Posting the same payload to Doc Library level works fine but it sets default on the content type itself which is not what I want:

https://{site}/_api/web/Lists/GetByTitle('Opportunities')/fields/getbytitle('OPNumber')";