question about opencore efi files

I have a question about this instruction:

After installing macOS, OpenCore Legacy Patcher should automatically prompt you to install OpenCore onto the internal drive. However, if it doesn't show the prompt, you'll need to either manually transfer (opens new window)OpenCore to the internal drive's EFI or Build and Install again and select your internal drive.

After I get through the install screens, I can not install OpenCore to the internal drive nor USB because the MacOS was installed to an external SSD using a MacBook and intent is to use the external SSD as a boot up drive for an iMac. After I get through the Ventura installation screens and skip installing openCore (to internal drive or USB) when I am prompted, what do I need to do next to manually add an EFI partition to the external SSD and copy the OpenCore build (for the iMac) from the installer USB over to the external SSD?