Creating functions

Hi,

When using a function that already exists in excel a small list comes up below as you type out the function of what you need to put in next. E.g =max(number1,number2...)

I have created a function in vba which requires 4 variables to be entered. The function (and variables needed) is this: DecayCorrect(isotope, initAct, initdate, desiredate)

it is working fine and if you press the fx button in order to get it up you then select each variable in turn and you can see the name of the variable you need. However if you click in a cell and type in the name of the function and an open bracket it does not provide the list as it does for pre-existing functions. Is it possible to get it to do this?

Thanks