Well, this is a function of Bigbluebutton Integration, a Nextcloud app. So obviously it requires installation and injecting a presentation is only possible through this app.
I am sure that the new feature of BBB2.6 is what you need, but you cannot use it from a client as it is. I've never used Wordpress, but I guess you need to pass two parameters uploadExternalDescription and uploadExternalUrl when you launch a BBB meeting to enable this function.
However there are at least two problems:
1, BBB2.6 is still in the process of beta testing (although quite stable already)
2, the Wordpress BBB plugin is not really actively developed. So you need to add some code by yourself to make the plugin to pass the two parameters.
As far as I look the code, the injection functionality of Bigbluebutton Integration app is not using the new two parameters. So you can use it with the current BBB2.5 just right now.
I hope somebody else who knows more answers...
Hiroshi