In your IDE there will be a memory map available through some linker file. It will contain all addresses in the program. Read the MCU manual to see at which addresses there is valid memory for your purpose, then reserve some of that memory for your variable. You have to read the documentation of your specific development platform.
Next, please note that it doesn’t make much sense to map variables at specific addresses unless they are either hardware registers or non-volatile variables residing in flash or EEPROM.
If the contents of such a memory location will change during execution, because it is a register, or because your program contains a bootloader/NVM programming algorithm changing NVM memory cells, then the variables must be declared as volatile. Otherwise the compiler will break your code completely upon optimization.
The particular compiler most likely has a non-standard way to allocate variables at specific addresses, such as a #pragma or sometimes the weird, non-standard @ operator. The only sensible way you can allocate a variable at a fixed location in standard C, is this:
#define MY_REGISTER (*(volatile uint8_t*)0x12345678u)
where 0x12345678 is the address where 1 byte of that is located. Once you have a macro declaration like this, you can use it as if it was a variable:
void func (void)
{
MY_REGISTER = 1; // write
int var = MY_REGISTER; // read
}
Most often you want these kind of variables to reside in the global namespace, hence the macro. But if you for some reason want the scope of the variable to be reduced, then skip the macro and access the address manually inside the code:
void func (void)
{
*(volatile uint8_t*)0x12345678u = 1; // write
int var = *(volatile uint8_t*)0x12345678u; // read
}