What are valid values for 'CoordSysRefCode' / application to geotiffwrite

I need to write a .tif in UTM.

I think that I need to use geotiffwrite(), but I have to use 'CoordSysRefCode' I can't tell what the valid values are. The references in the documentation page are broken links.

A bit of context:

I am reading in a tif file (created by QGIS) with geotiffread, manipulating the image, and trying to write it out with geotiffwrite. It's possible that the information is there from geotiffinfo() ... but based on other users' questions, it's not obvious how.

I'm using 2016b, but I can't get it to work in 2019a either.