how to use the API QP_PREQ_PUB.PRICE_REQUEST to request a unit price of an Item on a Blanket Purchas

Can anyone please help me with the script on how to use the API QP_PREQ_PUB.PRICE_REQUEST to request a unit price of an Item on a Blanket Purchase Order. Following is the script I was using which does not return any value and I am not sure if this is an issue with the API, Setups or the parameters I am passing to the API. Any help will be greatly appreciated.

Declare
p_line_tbl     QP_PREQ_GRP.LINE_TBL_TYPE;
p_qual_tbl     QP_PREQ_GRP.QUAL_TBL_TYPE;
p_line_attr_tbl    QP_PREQ_GRP.LINE_ATTR_TBL_TYPE;