How to write Expression Builder in Sandbox for user based

Summary:

How to write Expression Builder in Sandbox for user based

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We have a requirement to make PO Charge Account while creating PO editable only for one user "Buyer Custom" and for other users, it should be read-only.
I checked it through Sandbox>Kff Component>Read Only
We need to write Expression Builder to make it work

I tried writing following query
{securityContext.userInRole['BUYER_JOB_CUSTOM'] ? false : true}

But its not working
I have 2 questions:-
(1) Whay query we can use?
(2) In role, should we use Role Name or Role Common Name fom Users?