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(1) Check whether you are using GDI or GDI+. (2) In case you are using GDI, here is the documentation for the function you can use to paint a rectangle (but not filling the inside with color):
BOOL Rectangle(
HDC hdc,
int left,
int top,
int right,
int bottom
);
https://docs.microsoft.com/en-us/windows/desktop/api/Wingdi/nf-wingdi-rectangle
Example usage:
https://stackoverflow.com/questions/16159127/win32-how-to-draw-a-rectangle-around-a-text-string
To confirm that your application uses GDI, search your source code for a data type called HDC. This data type is a Handle to a Device Context.
Selecting the color and thickness for the rectangle border is slightly complicated in GDI programming. You need to create a GDI Pen object with the color and thickness, then temporarily swap (or "select", which is a terminology with special meaning in GDI) your Pen with the currently-in-use Pen in the HDC, then issue the Rectangle command to the HDC, and finally restore the original Pen object.
It is best to get a book "GDI programming with Windows 32 API". Without a book, it is hard to learn.
