The plugin may hlep you add contacts to Zoho Campaigns.
Activate the plugin then you will see a widget named "BSK Zoho Campaigns Widget" in widgets area.
Visit Settings->BSK Zoho Campaigns to set the Zoho Campaigns Authentication Key and Zoho Campaigns Mailing List Key.
Drag the widget to the sidebar where you want to show form which let users subscribe you campaign.
Visit https://www.zoho.com/campaigns/help/using-apis.html for how to get Authentication Key and Mailing List Key.