I am able to filter the accounts with condition startswith(userPrincipalName,'Priv_') but is not filtering with NOT condition i.e. as per business requirement, I have to filter the accounts whose prefix does not start with "Priv_". Please provide any example with Not condition.
You can try to use ne but I believe it will check for the exact match and not the prefix.
userPrincipalName ne 'abc'
Please note, this filter should work with your Azure REST api as well, so best would be to try in postman first and see if ne and startswith work together to achieve your requirement.