It's easy to start a campaign for subscribers. You simply go to the "Campaigns" section in the sidebar of YTMonster.

Hereafter, you will need to press the "New Campaign" button.

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", and in the amount field, you enter the amount of subscribers you would like to receive.

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.