For what it’s worth: my understanding and experience is that with /e/OS installed (which I think is how your phone came), you don’t need to enter a code from Fairphone’s generator page to enable unlocking the bootloader, despite what the Fairphone support site says (and indeed the /e/OS install instructions).
My belief is based on (a) this forum post, (b) my own experience – when I tapped “OEM unlocking” in the developer options of my /e/OS install, I was prompted for my own regular phone PIN, but not for any other code. After this, fastboot flashing get_unlock_ability says 1. (I haven’t gone any further yet, because I’m waiting for a new /e/OS release, so I’ve not personally verified that the bootloader is unlockable.)
(I think this doesn’t directly help you, because I don’t think you get as far as booting any OS. But I guess it’s helpful to know that it’s not inherently necessary to have somewhere to insert a code from the Fairphone site. I don’t know anything about the underpinnings here or why it’s OS-dependent.)