Perform TABLESAMPLE with WHERE clause in PostgreSQL
tablesample is an "attribute" of a table, not a query. So you need to write it write after the table name:

select * 
from customers tablesample system (1)
where last_name = 'powell';


Note that the where clause will be applied after the table has been sampled. It will not return 1% of all customers that have the last name 'powell'. But instead it will apply the filter on the sampled table.