What is HMODULE?

The method that you propose will work fine.

It seems that you have injected a dll into a target process and wish to obtain the address of a function in that dll in the target process from the process that injected the dll.

I assume that you also have the dll loaded in the process that injected the dll into the target process and that you want to create a remote thread in the target process and get it to execute the target function in the target process.

Since the dll that you have injected may not be loaded at the same address in the target process as it is in the injecting process you cannot simply use the address that you would obtain from calling GetProcAddress on the function in the injecting process.

An HMODULE is just the DLL's base address (see this answer for details). So you can take the HMODULE of the dll in your injecting process and subtract it from the address returned by GetProcAddress on your function. You can then add the HMODULE of the injected dll in the target process to this offset to get the address of the target function in the injected dll in the target process. Assuming this function has the correct signature, pass it as the thread function to your call to create the remote thread and you are now running the target function in the target process.

I explain this in more detail in this answer.

Call GetProcAddress. The offset cancels out, as you'd have to both add it (to get to the function) and subtract it (to get the base address), so you might as well not bother.