Follow the steps to pass the value.
1) Go to CustVoucher.post() method
2) Add code as below
myledgerJournalTrans = common as LedgerJournalTrans;
_custTrans.myfield = myledgerJournalTrans.myField;
3) Post the Journal and check the custTrans table
Follow the steps to pass the value.
1) Go to CustVoucher.post() method
2) Add code as below
myledgerJournalTrans = common as LedgerJournalTrans;
_custTrans.myfield = myledgerJournalTrans.myField;
3) Post the Journal and check the custTrans table
Rajpal's view on D365FO 
this is not working sir.could you please tell me how to achiev this
Maybe you need to add field to CustPaymManTrans and then use the same code.