Salesforce - log a call - want to add disposition

On the salesforce log a call window (the window that is installed as part of the RingCentral app that is installed in a Salesforce org) - - I need to add an additional field that is on the ActivityHistory record. (That's the record that the window writes to).

My Question is - is there a way to modify this page? Is there a way to write my own custom page? Are there any experts that have been into that portion of integration?

Thanks