Substituting your code and interface into someone else's program. How?

Question

C++ / C#

For example, the program is written in c++.
There are no sources.
The interface has fields for the full name, I need to fill them in by searching referring to the database (any external one),
i.e. I would like to add a search field in this database and a button to start the search and display them on the form of the program.
It turns out that you need to somehow implement your code for accessing the database, implement the code for creating an interface and filling in data.
In which direction should I dig?
ps I understand that it looks like viral technology, but that's not the point at all.

Answer 1

The simplest solution that comes to mind is to write your own small program with an input field for searching in the database, and after the search, it fills in the full name in that program.
You can start with the Spy++ utility to analyze the program with full name fields.
If you still want to tweak the interface and code of the program, then it already depends on what is used for the GUI (pure WinAPI, WinForms, WPF) and this is already dancing. It may turn out that it is written in the .NET Framework and you can easily disassemble and rewrite it in the studio, or you may have to implement your own DLL library with additional logic.

Answer 2

The simplest way is to emulate user actions.
The most popular solution is https://www.autoitscript.com/ (also provided as a library, there are wrappers for different languages).