Like Darth mentioned,
It should be fairly simple depending on how you set things up.
First, create the UMG widgets that will take the command - say a textbox for commands and a button for running the command.
Inside the UMG editor, click on the “On Release” event for your button (bottom right corner) to create an event that will fire when the button is released.
From this event you will then call another function / event inside your Character BP that will execute the command.
Button Pressed -> Use a switch on string node to have actions for each possible command (this means no complex commands). Cast to CharacterBP and call function fnJump
fnJump -> Makes character jump by calling Jump node (see thirdperson template for how this is done).
This is a very simple setup, but it also limits you because you are not really “interpreting” the commands. Instead the user has to know the exact command to use.
Hopefully this puts you on the right track!