Hi @Jayde F ,
According to my research and testing, you can follow these steps to create Modal Dialog Pop Up In SharePoint Online:
1.Create a new Wiki page.
2.Click Inset ->> Web Part ->> Media and Content ->> Script Editor ->> Click Add
3.Click Edit Web Part ->> Click EDIT SNIPPET
4.Input the code in the attachment, then click Inset and OK .(Please see attachment )
My test result:
More information for reference: Create Modal Dialog Pop Up In SharePoint 2016 And Office 365
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