Either way is ok with me.

If you want it based on selecting a cell, you could use a worksheet event:

Rightclick on the worksheet tab that should have the behavior.  Select view
code.  Paste this in:

Option Explicit
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
   If Target.Cells.Count > 1 Then Exit Sub
   If Intersect(Target, Me.Range("a2:a99")) Is Nothing Then
       Exit Sub
   End If
   
   UserForm1.Show
   
End Sub

You'll have to adjust the range and the userform name.

Or something like this to use an inputbox:

Option Explicit
Private Sub Worksheet_SelectionChange(ByVal Target As Range)

   Dim myStr As String
   
   If Target.Cells.Count > 1 Then Exit Sub
   If Intersect(Target, Me.Range("a2:a99")) Is Nothing Then
       Exit Sub
   End If
   
   myStr = InputBox(Prompt:="what do you want to enter in " _
                        & Target.Address(0, 0) & "?")
   
   If Trim(myStr) = "" Then
       'do nothing
   Else
       Me.Unprotect Password:="hi"
       Application.EnableEvents = False
       Target.Value = myStr
       Application.EnableEvents = True
       Me.Protect Password:="hi"
   End If
   
End Sub