The "what" here is that table based value lists that enter number from field 1 such as an ID code don't auto-complete. However, with a bit of creativity and scripting, you can set up a drop down list that enters a "Name" and thus can be set to auto-complete, can be used to look up the corresponding ID number and link your record by that ID value. See this demo file:
FileMaker 12 users: https://dl.dropbox.com/u/78737945/SimpleNameLookupDemo.fmp12
Pre FileMaker 12 Users: https://dl.dropbox.com/u/78737945/SimpleNameLookupDemo.fp7
The scripting, BTW, is there to handle cases where you get two or more records with the same name but different IDs.