[Threshold]

When a player chats, OnPlayerText is called. So that is the callback we want to use.

pawn Код:

public OnPlayerText(playerid, text[])
{
    //Code
    return 1;
}

We want to apply the chat animation, which is "IDLE_chat" in the "ped" library.

pawn Код:

ApplyAnimation(playerid, "ped", "IDLE_chat", 4.1, 1, 1, 1, 0, ((strlen(text) * 100) + 1000), 1);

(strlen(text) * 100) + 1000) is used as the 'timer' parameter in ApplyAnimation. This means that the animation will play for a certain amount of time, so with this, the longer the text you typed in the chat, the longer that your player will appear to be 'chatting', adding a more realistic effect to it.

The full code:

pawn Код:

public OnPlayerText(playerid, text[])
{
    ApplyAnimation(playerid, "ped", "IDLE_chat", 4.1, 1, 1, 1, 0, ((strlen(text) * 100) + 1000), 1);
    return 1;
}

References:
ApplyAnimation
OnPlayerText