Plugin for managing subscriptions to a mailing list. It provides a simple form for subscription to your mailing list through single or double opt-in.

Noted That:

To install correctly this newsletter-subscription-widget-for-sendblaster.zip.
Fisrt Download the newsletter-subscription-widget-for-sendblaster.zip to your computer
Extract/Open newsletter-subscription-widget-for-sendblaster.zip to Your Computer.
Then, Find readme.txt file inside newsletter-subscription-widget-for-sendblaster.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "newsletter-subscription-widget-for-sendblaster.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "newsletter-subscription-widget-for-sendblaster.zip" Downloaded plugin from your computer, Where you downloaded newsletter-subscription-widget-for-sendblaster.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Newsletter subscription optin module" folder is added on left sidemenu. Now, Click on "Newsletter subscription optin module" folder.

Noted that: If you do not see "Newsletter subscription optin module" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Newsletter subscription optin module Configurtions and Settings or How to work "Newsletter subscription optin module" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Newsletter subscription optin module" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Newsletter subscription optin module
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
This section describes how to install the plugin and get it working. Unzip the downloaded archive Upload the script `wpsb-opt-in.php` to the `/wp-content/plugins/` directory Activate the plugin through the ‘Plugins’ screen in WordPress Use the Settings->Plugin Name screen to configure the plugin Go to `Wp SendBlaster Opt-in` and set the email address that you intend to use for managing subscriptions. This should be the same address you have provided in your bulk email software Manage subscriptions configuration In the `Appearance->Widgets` menu, drag to your sidebar the newly added plugin

This section describes how to install the plugin and get it working.

Unzip the downloaded archive
Upload the script wpsb-opt-in.php to the /wp-content/plugins/ directory
Activate the plugin through the ‘Plugins’ screen in WordPress
Use the Settings->Plugin Name screen to configure the plugin
Go to Wp SendBlaster Opt-in and set the email address that you intend to use for managing subscriptions. This should be the same address you have provided in your bulk email software Manage subscriptions configuration
In the Appearance->Widgets menu, drag to your sidebar the newly added plugin

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Reffered: https://wordpress.org/