Unable to fetch Sales Order details from App composer trigger on Install Based Asset

Dear experts,

We are unable to fetch Sales Order details from App composer trigger on Install Based Asset using an standard SalesOrderHeaderVO. Could you suggest on this issue?

The following steps are performed:
1. Navigate to App Composer > ERP and CRM Cloud > Install Base Asset object > Triggers
2. Create a ViewCriteria with below details

def so= "547"
def vo = newView('SalesOrderHeader')
def vc = newViewCriteria(vo,' SalesOrderNumber = '+so)
def iter = vo.findRowsMatchingCriteria(vc,-1)
while (iter.hasNext()) {
def r = iter.next()
setAttribute('PO_Number_test_c',r.CustomerPoNumber)
}
3. In above script we are trying to fetch CustomerPoNumber(OrderHeader Attribute) and map with one IB asset DFF.