Pretty straight forward. Add an input field when the UI is up to write in a seed you'd like to generate a map from next time that the "Generate New Terrain" button is pressed. Perhaps place this field in front of the button, so it's obvious what it does. Also, when the button is pressed, empty the input field. This will make it clearer that the seed has been used, and it will be displayed anyway as the "current map seed".
Stretch goal.
Pretty straight forward. Add an input field when the UI is up to write in a seed you'd like to generate a map from next time that the "Generate New Terrain" button is pressed. Perhaps place this field in front of the button, so it's obvious what it does. Also, when the button is pressed, empty the input field. This will make it clearer that the seed has been used, and it will be displayed anyway as the "current map seed".