This post will provide you with the complete detailed method to upload Inventory Item category assignments into your instance through WebADI.
A complete functional solution to uploading category assignments, no IDs required whatsoever. Just enter all the functional data available to you (that the WebADI demands for uploading category assignments) and the WebADI will do the rest for you.
This setup can be used to both CREATE and UPDATE category assignments at Item level.
Here is the documentation to achieve the aforementioned:
In case you are interested in Uploading Master Items through WebADI, ive also prepared a WebADI to achieve that: