It's been a while since I've installed Elementary on anything. But, since the installer is very similar to the *buntus, there should have been a prompt in the lower half of the partitioning screen to request boot file placement. The typical default is something like "/dev/sda", but can be changed. It should be changed to the EFI partition (i.e., /dev/sda1). That is assuming the partitioning table was set to GPT and /dev/sda1 is the fat32 formatted partition identified as an ESP.