script task in process model

You need to run write to DSE node. NOT write to Multiple DSE. 
because you need the ID of first table in the second one. Therefore, you a write to DSE, store the output in the same variable, use a script task to map the id into another table, and then write the second table. 

Hi Harshit ,  I have inserted data to first table using Write to DSE smart service. Then used script task to reassign the inserted id  from the previous step to foreign key of the second table.  It asignes  foreign key value to the first record and doesn't assign other record.  . In the scrip i used  multiple parametr value and it didn't help

If you need to insert multiple record into the second table, you might want to use a!update() in a foreach() to update the foreign key in all the records.

1 ) Where should I  write that code in scripttask  ?  Is it output tab and expression window  ?

2)  below code looks okay ?

a!forech ( items : transactionrlist

expression : a!save( fv!item.Foreignkey , id)

)  

3) if above code is wrttien in expression editor window ( script task, output tab)  then   what is the value of target  ?

Your code in the output of a script task should like something like this

a!foreach(
  items: pv!something,
  expression: a!update(
    fv!item,
    "foreign key field",
    pv!customer.id
  )
)

a!save() does only work inside a saveInto on UI components.

Thank you ,Stefan .It is working now . 

Thank you ,Stefan .It is working now .