A random number generator is a tool that allows users to generate a random number within a specified range. This can be useful for many applications, such as generating random passwords, selecting a random winner in a contest, or simulating random events in a game.

To create a custom formula for a random number generator in Google Sheets using Apps Script, follow these steps:

- Open a new or existing Google Sheet.
- Click on “Tools” and then “Script editor” to open the Apps Script editor.
- In the Apps Script editor, create a new function called “randBetween” that takes two arguments: the minimum value and the maximum value of the desired range.
- Inside the “randBetween” function, use the “Math.random()” method to generate a random decimal between 0 and 1.
- Multiply the decimal by the range of values you want to generate (i.e. the difference between the maximum and minimum values).
- Add the minimum value to the result to shift the range of random numbers to the desired minimum value.
- Return the result as the output of the function.

Here’s an example code for a custom formula to generate a random number between 1 and 100:

```
function randBetween(min, max) {
var range = max - min + 1;
var randomNum = Math.floor(Math.random() * range) + min;
return randomNum;
}
```

To use this custom formula in your Google Sheet, simply enter “=randBetween(1,100)” into any cell where you want a random number between 1 and 100 to be generated.

This custom formula can be modified to generate random numbers within any desired range by changing the minimum and maximum values in the function call.