Use Custom Keyword In API Object

I’m trying to call a custom keyword that generates data from within a web API object (specifically a JSON PUT) I’m not sure how to go about doing this. For reference, all the other data is either boilerplate being set via global variables (the same for every request) or being pulled in from a data source. However, there is one field that must be unique for each transaction, so I wrote a keyword that generates a random string to fill it in.