Hi Andy,
try to use document.querySelector('.someClass').value = "Value you need"
eg: check the pics and oml, I add a Run Javascript Action from HTTPRequestHandler and add the script inside and add the class "inputTest" to be selected by the query Selector


Results:

Sometime use the getElementById is trick on traditional web, so I advice to do as I show in the pic
Hope I could help you
Best regards
Carlos Lessa