The retrieval system...
The below URL is a general format that has been implemented in other environments and is an out of the box solution for AX retrieval.
Servername = The name of the server where AX web is setup
Datasourcename = The name of the AX Datasource
ApplicationName = Name of the AX application retrieving from
Indexfield = field number of the index retrieving such as if "invoice number" is in field1 and want to retrieve by "invoice number" then enter field1.
IndexValue = the value of the Index field you wish to retrieve. For instance invoice number = 12345 would be field1=12345 in the URL above.
Add additional Indexfield using the & symbol and enter necessary values for retrieval.
I've mocked up the url below further to emphasize the changes needed to make the URL work.
Above I specified an additional index field number. In my example field5 is a doctype and Invoice is the specific doctype to match with the previous Invoice number (field1) of 123456.