Is this same solution for adding the contacts in the Contacts module to the email marketing mailing list contacts? Need help there. 

Here is one solution: Add contacts to mailing lists using Automated Actions (discussed in this question and answer)

Or there's an add-on for Odoo 12:  https://apps.odoo.com/apps/modules/12.0/mass_mailing_partner/

hello, i'm also finding a way to use my contacts in mailing lists. if you've found out, can you please share it.