how to manually add backup files to iTunes; restore iphone on new computer

I broke the screen on my iPhone 6, which was paired and backed up on an old Macbook running an older version of iOs (is not able to upgrade to Mojave).

I ordered a new iPhone X, but this will not connect to the old Macbook since the version of iTunes is outdated and cannot be updated.

I have a desktop Mac which is new and up to date, but the iPhone 6 was never paired with this nor backed up. I cannot do this now since the screen is broken and I cannot click "trust" this computer.

I do however have the backup folder transferred to this desktop Mac computer. How do I get iTunes to manually recognize the backup folder and then use this to restore and set up the new iPhone X?