It's easy to start a campaign for subscribers. You go to the Campaigns section in the sidebar of YTMonster.
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Hereafter, you will need to press the New Campaign button.
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Once you're on the New Campaign page, you simply enter your YouTube channel URL in the first field. In the dropdown, you select "YouTube Subscribers," then you press "Next".

In the number field, you enter the number of subscribers you would like to receive.
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After entering all the information, you can submit the campaign as long as you have sufficient credits. If you've done everything correctly, a campaign for subscribers should now be Live in the Campaigns section.