To edit the commands message (This is the message that contains all the commands that is put in chat when the player uses the /commands) go to the BP_Chat widget then go to the graph view and in the class defaults find the Help Message.
Here is where you set the messages text as well as add new instructions for players to use your commands.
It uses rich text which allows for different fonts and style text within the same text box. By default the command part of the message is surrounded by <Command> then ends with </> like the example below.
<Command>/quit /exit</> to close game
This means that the text /quit /exit will use the command font settings that are set in the ChatGroupsGuildsRichText in the content browser, the to close game text will use the default row from the ChatGroupsGuildsRichText . Select the Command row which allows you can change it if you want to use different colors, font size or font.
So if you where to add a new command you would add to the bottom of the Help Message text. You will want to use Shift + Ctrl to go to the next line in the message before adding your new command text otherwise it will be on the same line as the previous command.
<Command>/new command</> to do your new command thing
And the total help message would look like the image below.