Is it possible?

[mystic herald]

so is it possible to create a placeholder that would register any message a player writes?

[warped topaz]

I'm not exactly sure what you mean by registering?

[mystic herald]

ik a bit late but I ment so like it would understand if a player writes a message so like I would be able to put placeholders/text before/after it so like [message] and like placeholder for the [message]

[warped topaz]

So you want to customize every message a player send by appending a placeholder (or multiple placeholders) to it? Then you'll have to use a chat formatting plugin

One that supports PlaceholderAPI placeholders