Display the IMG with transaction code SPRO
Then dispy either the Reference IMG or your project IMG as appropriate.
Enable the IMG Activity Key by selection Additional information --> Additional Information --> Display Key --> IMG Activity as shown below
Find your configuration item in the IMG the normal way.
The transaction code is usually, but not always the last four characters of the IMG Activity key. In the example below, the transaction key for the IMG task Maintain terms of payment is OB88
I say usually it is the last four characters because this isn't always true. If you look at the the items for "Define Cash Discount Base for Outgoing invoices" or "Define Tax Accounts for Outgoing Invoices" the last four characters are not the transaction code. In this case, the transaction codes are OB70 and OB40 respectively, so some trial nad error may be necessary to identify the code.