USER form..need to use to pass a variable to a cell reference and back.

Have an excel workbook that is owned by someone else..I cannot change the essential essence of the book, however the interface is VERY user unfriendly and displays terrible.
I have created a form in workbook to enter data, however i am stuck bec I cant figure out the code to have the form take an input, place it in a cell reference within the workbook, and then pass another result back to the form(calculation result)...the formulas are so convoluted, I cant repeat them..i just need to get cell references, in and out of book..Can someone help?

Welcome to the forum!

Private Sub UserForm_Initialize()
  Range("A1").Formula = "=Sum(A2:A3)"
End Sub


Private Sub CommandButton1_Click()
  Dim r As Range
  
  Range("A2").Value = Val(TextBox1.Value)
  Range("A3").Value = Range("A2").Value + 1
  TextBox2.Value = Range("A1").Value
  
  Set r = Cells(Rows.Count, "A").End(xlUp).Offset(1)
  MsgBox "Next empty cell in column A: " & r.Address, _
    vbInformation, "Cell Address"
End Sub