You need punctuation, desperately; almost run out of breath reading that run-on sentence 
What you’re asking for is somewhat involved, I’ll just outline what’s needed briefly.
For the widget:
- create a widget with the prompt - a text box showing which button to press will do
- override onKeyDown and check what button is being pressed
- make a list with the correct key sequence, an array perhaps or a Map
- once the correct key has been pressed, step to the next element in the array
- once the last key in the sequence has been correctly activated, have the widget send a notification to whatever needs to act next
- if any of the sequence steps fails, have the widget communicate that as well
- switch back to Game and UI mode (see below)
- remove the widget from the parent, destroying it
For the trigger:
- onBeginOverlap create the widget
- add it to the screen and set position in the viewport
- switch to UI only and set the focus to the widget created above (this way the player cannot do anything else but mash buttons)