Question: I would like to ask some help on how I can get the Mod 10 check digit of Contact.DonorID.
the result should be

```
Contact.CRN__c = Contact.DonorID + CheckDigit

```

Just like this:

> 
> Check Digit is **2**
> 
> 
> CRN: 0018457**2**
> 
> 
> 

It is my first time to do it and I don't have any idea, I've been struggling for 3 days searching about it. I found this link -- [Luhn algorithm check digit in formula field](https://salesforce.stackexchange.com/questions/79536/luhn-algorithm-check-digit-in-formula-field) but it is using formula field, so, it can't help me.

Thank you in advance!

Answer: 
I found that to add a button to a list view, navigate to "Search Layouts for Salesforce Classic" (right below Search Layouts) and add the button to the List View Layout there.

Note that the button will not appear in the "Recently Viewed" list view, but it will appear in all the other list views.
Setup > Object Manager > Case > In the sidebar > ListView Page Layout > Edit List View then in the custom button section add your list button, And Save it.

You see the list button on the list views other than recently view.