To add a goal to a Number widget, first determine if the goal aims higher or lower than your current performance.
An example of a higher goal is "MRR", as you usually want it to grow (as much as possible). So, if your current MRR is $250K, your goal could be $275K (more than $250K).
An example of a lower goal is "First Reply Time", as you usually want it to be as small as possible. So, if your current First Reply Time is 30 minutes, your goal for it could be 25 minutes (which is less than 30 minutes).
Higher goals
Use the widget's config menu to set a higher goal by clicking Set a goal and entering the target value as a Goal.
Lower goals
Use the widget's config menu to set a lower goal by clicking Set a goal and entering the target value as a Goal (up until here, it is identical to higher goals).
Then use the starting from option to define the current performance of the metric; this will change the direction of your progress, so the lower it gets, the closer to the goal it is.