To create a custom property that sums the length of characters in two event properties while handling undefined properties as 0 characters, you can use a combination of the len() function and the defined() function in Mixpanel's custom property formula.
Here's a formula you can use:
```
(if(defined(properties["first_property"]), len(properties["first_property"]), 0)) +
(if(defined(properties["second_property"]), len(properties["second_property"]), 0))
LBKSThis formula works by:
1. Checking if each property exists using the `defined()` function
2. If the property exists, it calculates its length using the `len()` function
3. If the property doesn't exist, it uses 0 instead
4. Finally, it adds the two values together
The `len()` function in Mixpanel returns the length of a string or list, as documented in the custom properties reference [Mixpanel Custom Properties Functions](https://docs.mixpanel.com/docs/features/custom-properties#functions).
You can create this custom property by clicking the plus button to the right of the search bar in your Mixpanel report, selecting Event Property, and then entering this formula in the formula field. Remember to replace "first_property" and "second_property" with the actual names of your event properties.
