What does it mean?
I'm not sure what the first game to do this was, but a "tell" is a private message sent to another player. I think it originates way back from text-based MUDs where to message someone you'd type something like
tell <name> <message>.
These days many multiplayer games call those messages "whispers" instead, but often
/tell will be aliased to it. For example, in World of Warcraft you can message someone either via
/w for "whisper" or
/t for "tell".
So, "please send tell" basically means "contact me privately to respond".
Another meaning is "Pacific Standard Time," a time zone eight hours west of Greenwich, used by places like Los Angeles (movies, games,) San Francisco (start-ups, games) and Seattle (Microsoft, games.)
If you're talking about scheduling of events, then this is a much more likely meaning of the acronym.