XML or txt Sitemaps for google discoverability

Having an XML sitemap plays a big part in the discoverability of our content on google. But we can't seem to add one, or even add a simple.txt sitemap. 

We can generate the sitemap from 3rd party suppliers but we need ontraport to provide the ability for us to upload it.

If it's not important then why does ontraport have a sitemap? https://ontraport.com/sitemap.xml

Can we please make this critical sitemap indexing available please.

Cheers,

Nils