initialize and use variable in query

All,

I need to initialize a variable and then use it in query. Ultimately this
will part of a recursive CTE but for now I just need to work this out. I
followed the docs and thought I needed something like this. But does not
work-maybe I have misunderstood. Is this possible?

SET max_parallel_workers_per_gather = 8;
SET random_page_cost = 1;
SET enable_partitionwise_aggregate = on;
Do $$
Declare startdate date;
BEGIN
startdate := (select max(fctrdt) from fnmloan);
END $$;
select
fnmloan.loanseqnum
,fnmloan.currrpb
from
fnmloan
join
fnmloan_data
on
fnmloan_data.loanseqnum = fnmloan.loanseqnum
where
fnmloan.fctrdt = (select * from startdate)

limit 10