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If you have custom URLs that are not in the CMS, you can manually add them to their own Sitemap Index via Site Settings → Sitemap.
Resources:
Links: Plugin store page: https://plugins.craftcms.com/seomatic
https://nystudio107.com/docs/seomatic/resources.html#crafting-modern-seo-presentation
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Resources shared by Engineering:
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Repository: https://github.com/nystudio107/craft
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