For FI accounts the search string will work as expected.
But this has only an effect on posting area 1, if IHC has been registered for posting area 2. So IHC will execute this posting according to customizing in transaction FIBF (event 2810 and process 2820).
IHC gives you 2 ways of determining the IHC accounts for subledger posting:
1. Account determination from payment notes (TBKKIHB1).
2. Account determination from external accounts (TBKKIHB6).
Refer to the documentation of the following IMG customizing node for more details about this topic:
-> Financial Supply Chain Management
-> In-House Cash
-> Account Management
-> Payment Processes in In-House Cash
-> Central Cash Receipt / Incoming Bank Statements
-> IHC Account Determination from Payment Notes.