The usual "Add-in" option in Excel has XL Miner Toolpak, but that's not the Data Analysis Toolpak. The Add-ins for Excel also refer to the Microsoft Store, which is blocked.

This must be done in the standalone version - not available in the web version.

Windows: In Excel, File (or Home) -> Options -> Add-ins -> Go (button) -> check Analysis Toolpak box (or Solver), click OK, restart Excel.

Mac: Open Excel new/existing document -> Data tab -> Analysis Tools gear icon on right -> check box for Analysis Toolpak (or Solver).

Here is a youtube video of the process - this video might be taken down at some point, so please refer to the written process above if it's gone:  https://www.youtube.com/watch?v=mIoS7IRo36c