Head to ''Campaigns'' in the main navigation bar at the top of your account and select ''Campaigns'' from the drop-down. Navigate to the campaign (event), and click on it by selecting the small pencil icon on the right-hand side.
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The ''Edit Campaign'' screen will open up and you'll want to head to the ''Fundraiser'' tab.

Scroll down and the Custom Fundraiser link can be found under the section titled ''Custom Fundraiser Link''.
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Want to create an event / 1-click custom facebook fundraiser link for inviting supporters to create a Facebook or JustGiving fundraiser? You will need to create a challenge campaign.