Yes, providing you have the email journeys tool enabled in your account, you can contact supporters who have registered and created a JustGiving fundraiser via the GivePanel/ JustGiving integration.

Please find more information on GivePanel Email Journeys.

Our email tools work for all registered supporters, including those who have created JustGiving fundraisers.

As well as the standard email triggers and conditions available in the email journeys tool, you will also have the option to segment your registrants by those who have created a JustGiving Fundraiser specifically and claimed or not claimed their JustGiving fundraising page.
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You can also include the shortcode [UnclaimedPageURL / ] in these emails to generate claim links for supporters who created a JustGiving fundraiser but haven't claimed it yet.