Set Items template When a Opportunity is created

I want to autoset a list of Items when a new opportunity is created in order to the sales Representative doesnt have to create the lines, when the entity field of the opportunity is set to certain value;i have try using the nlapiSelectNewLineItem function and deploying this record as an user event script beforeload

function SetCreditItem(type)
{
nlapiSelectNewLineItem('item' '1');
nlapiSetCurrentLineItemValue('item', 'item', '9');
nlapiSetCurrentLineItemValue('item', 'custcolcathay_plazo', '12');
// commit the line to the database
nlapiCommitLineItem('item');
}

I dont know if I can  be trigger the beforeload function to work when the user access the create a new record form, any guidence or suggestion will be appreciated