Head over to your reply account.
Settings > Integrations > API - or click on this link

Then head over to the custom fields section in Reply and add add the following custom fields
EMAIL1, EMAIL2, SELECTED_EMAIL, BACKUP_EMAIL1, BACKUP_EMAIL2 (this can you can choose whichever you'd like)

After you've added it, it'll show up like this

Once done, copy your API key and then paste it in the integrations section of Smartwriter

Select Reply and then paste in the API key you've copied from Reply, Name the API key so you can easily identify it (name it whatever you want)