How do I write a macro that opens up the single form data upon AfterUp

[MinL]

Hi,
How do I write a macro that opens up the single form data upon AfterUpdate?

For example:
My first entry field is called ID# on the form. So, supposedly I am entering
a new data entry, as soon as I entered the information for ID# (which is
AfterUpdate), I would like to open up the file that will having matching ID#.
So, how do I write a macro that will open up a form that already contains the
ID#? If it does not that ID# already in the database, then I don't want that
macro to run.

Please help, thank you very much in advance.