To begin with, there is an issue with your form that it will not accept any [Event Procedure] in the On Current event. I did not dig into it but rather created a brand new form which ended up working.
The proper way to call your function is:
Me.txt8 = basMinsToHrMnSec(DateDiff("n", RequestTime, ResponseTime))
However, this same calculation can be done without a function, like this:
Me.txt8 = Format$(CDate(ResponseTime) - CDate(RequestTime), "hh:nn:ss")
Hope that works for you.