As far as I know, there is no such option to allow users request add-ins.
If you want users to request add-ins from https://appsource.microsoft.com/en-us/marketplace/apps?product=office&page=1&src=office&corrid=442d14b6-aee5-9325-691a-f50a44a6ae25&omexanonuid=&referralurl=, this may require additional development work, such as on SharePoint.
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