the 'alert()' function is client side API and wont work in a script include (server side). And your script include class is 'CustomtestClientSI' where in the UI script you have 'CustomDocuSignClientSI'. change one or the other to match.  I suggest you review OOB UI Scripts that use GlideAjax to see working examples. the UI Script named 'IncidentalsCalculator' is a good example.