The documentation says you have to use a where as a querystring:
SQL WHERE condition for filtering the loaded data. The parameter only allows the following types of SQL syntax:
column names, values and basic operators: =, !=, >, <
AND & OR operators, parentheses
column BETWEEN value AND value
column LIKE value
column IN (values)
column IS NULL
NOT keyword for the above expressions (NOT BETWEEN, NOT LIKE, NOT IN, IS NOT NULL)
Other expressions and SQL functions are not supported.
Example: ~/rest/cms.user?Where=UserIsEditor=1