The FormulaArray property requires R1C1 notation (according to Excel Help)

I think this will point you in the right direction:

Sub PutArrayFormula()
Dim cCell As Range
Dim sFormula As String

Set cCell = Range("A1")
sFormula = "=SUM(" & Range("B2:C2").Address(ReferenceStyle:=xlR1C1) _
    & "*" & Range("B3:C3").Address(ReferenceStyle:=xlR1C1) & ")"
    
cCell.FormulaArray = sFormula

End Sub
Alternatively, if you're fluent in R1C1, you could use this line in the code:
sFormula = "=SUM(R2C2:R2C3*R3C2:R3C3)"
Does that hlep?