Question How can I get sever to output string from player? (PLEASE HELP!!)

I am about to lose my mind with this.

I have a simple game set up where there is a host and one client on different machines, local connection. I am logging a string of data (coordinates of a networked game object) as a .csv file. It outputs a file for each connection its own device (super simple!). I now need to run the client as in WebGL on the same device as the host, but still need to log the data string to an output file. Obviously I can't write to disk using WebGL. I thought it would be simple to somehow send the string to the host, and have the host output both the host data and client data to single .csv file... but I can't for the life of me figure it out.

PLEASE HELP! Or any other suggestions on how to set this up would be amazing.