The essentials document says:

"Allow commands in kits and use {player} to replace with player's name."

But how can I set a command in a kit?

read the wiki

The wiki doesnt even explain how to use commands in the essentials kits...
It just says "Allow commands in kits and use {player} to replace with player's name."
But how do you add a command in the essentials kits...

Sorry, thought it was a bit more detailed.

Just put the command in there so like /give {player} diamond 1