Office 365 Subdomain Setup

You are not going to get that level of control. User can login into portal.office.com, when they enter their upn/email address it will recoginize their email domain and redirect them to the correct O365 tenant. When you create your tenant, you primary domain will be used to create the SPO url, in your case, mynonprofit.sharepoint.com will be the url for all of the spo sites and the new sites will be created in either the mynonprofit.sharepoint.com/sites or mynonprofit.sharepoint.com/teams paths. You cannot create your own paths or change the root SPO address after your tenant has been created, so be careful when you are choosing your tenant name. You can add multiple email delivery domains, however, only the first one is used by SharePoint.

Here is an Domain FAQ that may be helpful.

https://support.office.com/en-us/article/Domains-FAQ-1272bad0-4bd4-4796-8005-67d6fb3afc5a?ui=en-US&rs=en-US&ad=US