Prepopulate or Clone field values using Custom Button or Custom Link in Lightning Salesforce

Salesforce announced to create custom buttons and links that pass default field values to a new record beginning from Spring'20 release.

To construct a custom button or link that launches a new record with prepopulated field values, use this sample formula:

1. /lightning/o/Account/new?defaultFieldValues=
2.     Name={!URLENCODE(Account.Name)},
3.     OwnerId={!Account.OwnerId},
4.     AccountNumber={!Account.AccountNumber},
5.     NumberOfEmployees=35000,
6.     CustomCheckbox__c={!IF(Account.SomeCheckbox__c, true, false)}

Note:
The URLENCODE function works only when creating custom buttons and links. You can’t use it for custom fields.

Reference:

Spring 20 Release Notes 1

Spring 20 Release Notes 2