Hi @tylor92 ,
As per your description, as phpBB web forum is a third-party and after search a lot I’m afraid currently it's not possible in SharePoint online environment, here I found thread in phpBB forum Using phpBB in Sharepoint.
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From the perspective of SharePoint, there are two workarounds for your reference:
1, In SharePoint online we can only embed content from some websites by copying and pasting the site's web address for the content into the Embed web part in SharePoint online site. For reference: Display content using a website address
2, Go to SharePoint admin center, create a site-->other options-->more templates, then you will go to the page like below:
There is a Community Site template you can choose.
And as phpBB web forum is a third-party, it is recommended that you go to the phpBB forum to disassemble the function key discussion in order to get more help.
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
Thank you for your understanding and support.
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