New MRR is MRR contributed by an account in this month when the account contributed $0 last month AND has never contributed MRR in the past.
To calculate this metric on your own, your best bet is to use the Subscriptions export (Status: All, Time Range: All Time) and then check the account_code column for duplicates.
If there are duplicates, then this means that these accounts have contributed to MRR in the past since they have two subscriptions listed in the export. You would want to remove all duplicates.
Now that you have the remaining list of non-duplicates, you'd use the activated_at column to filter out all subscriptions that were NOT created outside of the date range you're analyzing.
For example, if you're looking at New MRR for March 2020, you'd remove all subscriptions with an activated_at date before and after March 1-31, 2020. The remaining list of subscriptions would be considered new MRR and you can use the Amount column to sum up the total.