I am trying to define a searchhelp for COBRB-KONTY. SAP uses a special function that reads customizing tables. How can I implement this functionality for my own InputField?

You can write your own searchhelp function.
Two examples are to be found in the documentation on InputField, section "Programmed matchcode help".

I suggest you proceed as follows:

- Find out which function is used in the SAP system to display the seachhelp (use ABAP debugging)

-Test the appropriate parameterization of this function with SE37, test environment

- Write an ABAP exit program like the one shown in Example 2 of the keyword documentation