Your interrupt handler needs to have the same name as defined in your toolchain, usually the startup file. Which is often assembler code.
In the generated map file, you should see your handler/address present in the respective vector table entry.
Toolchains often provide default handler for interrupt or fault handlers, which are declared as "__WEAK" and can be overwritten.
The handling of such linking issues is not identical across toolchains, so you might need to consult the Keil tutorials.