don't you have to divide the 8% by the number of payments per year (for example 12 for monthly payments, or maybe it would be 10 for this short loan) to convert it from annual rate (APR) to rate per period?

=PMT(8%/12,10,1000,0,1)

If you dont like the result being a neg number, you can convert the PV to a negative number...

=PMT(8%/12,10,-1000,0,1)