aexec <playerName> <command>
For example: /aexec Callum say Hello world!
Moderator note for knownledgebase:
This (example) is a reason why you should never trust what a player appears to say in chat, as admins can use aexec to speak out of anyone's name.
Unless you have access to the server.log, you cannot determine whether or not aexec was used to force a player to 'talk'.
aexec can be explained to mean 'admin-execute (a-exec)', in which admin will be able to execute commands for a player. Likewise, other commands that can be triggered using aexec can also make the player perform or seem to do actions they are infact not doing themselves. For that reason it's used commonly as a tool to troll players.
Maybe you have noticed not all commands can be triggered using aexec, it would appear only commands implemented through client-sided scripts allow for it to work. Hardcoded MTA 'commands' (such as "say" for mainchat, "me" for /me in chat) can generally be used, but commands such as "disconnect" or "quit" can, although hardcoded, not be triggered. Also you should pay attention to the fact any command in the aexec syntax cannot have a / (slash), just like how typing commands in F8 for yourself needs to be without slash. So it should be "aexec [exact playername] command" and not "aexec [exact playername] /command", and for "T" chat execution it should be "/aexec [exact playername] command" but still not /aexec [exact playername /command"