I just did a speed run for the zxspec, with the game river raid. However, there doesn't seem to be a section for that. Little help.

Ask the moderator of the game or ask in the game's forum.

I've attempted that with limited success.

The mod hasn’t been online in five days, so he wouldn’t have seen your post in the game’s forum yet. You can either wait for him to come back online, or you could message him through one of his social media.

Either way, be specific that you’re asking for a new platform to be added to the leaderboard. It’s not clear from your first post that that’s what you want.