[Urgent/Simple] I have a defined function and a defined range of cells. How do I do =MyFun

Here is what I have:

Dim Rngx2 As Range
Set Rngx2 = Sheets("Sheet2").Range("A1:A" & Range("A1").End(xlDown).Row)
Sheets("Sheet1").Range("A3").Formula = "=MyConcat(Rngx2)"

I want to use =MyFunct for that rng. How do I do that?

Something like... =MyFunct(rng) ... but it won't work.