Here let us open the SharePoint Online list, where you want to show the alert box. From the list ribbon (Classic site) or command bar (modern site) click on Nintex Forms button like below, which will open the Nintex Forms for Office 365 designer.
Then in the Nintex forms designer, click on Form Settings button like below:
Now Save it and then Publish the Nintex form to SharePoint Online site.
Now when you will open the list and try to add one item into that SharePoint Online list, it will show the dialog box on the form load like below:
You may like following Nintex forms for Office 365 tutorials:
- Steps to create column in Nintex forms designer in Office 365 SharePoint Online
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- How to implement mandatory field validation in Nintex forms in Nintex forms for Office 365 SharePoint Online?
- Steps to show cascading dropdown in Nintex Forms for Office 365 SharePoint Online
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- Add different layouts into Nintex forms in Office 365 SharePoint Online
- How to design multiple step forms using Nintex forms for Office 365 SharePoint Online?
- How to set a default value for lookup column in Nintex forms for Office 365 SharePoint online?
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51 JSOM Examples in SharePoint Online