Assuming your variable name as String_value
String_Value = “1”
Create new string variable for selector and in assign activity use the below.
string_selector = "<webctrl id = 'poDetailItemDatas["+String_Value+"].unitPrice' tag = 'INPUT'>"
If your value is not string then use (toString) to convert into string.
string_selector = "<webctrl id = 'poDetailItemDatas["+int_value.tostring+"].unitPrice' tag = 'INPUT'>"
Refer this xaml. sample.xaml (7.1 KB)