Macro to call-up UserForm

[Mr. G.]

Can anyone help me on creating a macro that would automatically call up a
UserForm when a particular row/cells (from an Excel Spreadsheet) are
populated rather than requiring the User to click a CommandButton?

Currently, I have a message box that pops-up from the following worksheet
event:

If Intersect(Target(1), Me.Range("Y53:BN53")) Is Nothing Then Exit Sub

If Intersect(Target(1), Me.Range("Y53:BN53")) Is Nothing Then Exit Sub
If Target(1).Value >= 250 Then
If MsgBox("Was vehicle held over the weekend?", _
vbYesNo) = vbYes Then
MsgBox "Please give reason with approving Manager's name"
End If
End If
End Sub

Also, I would like for the User's entries (i.e., "reason for the weekend
rental" and "Approving Manager's Name") to appear in cells/columns Y104 and
AB104, respectively.

Thank you in advance.