You don't need an input box. That's what the text box is for.
Add two command buttons to your userform - 1 to quit and the other to change the value.
Code the userform as follows
Option Explicit
Private Sub CommandButton1_Click()
If TextBox5.Text = "" Then
TextBox5.Text = TextBox4.Text
End If
Selection.Hyperlinks.Item(1).TextToDisplay = TextBox5.Text
Unload Me
End Sub
Private Sub CommandButton2_Click()
Unload Me
End Sub
The main code loop is as follows
Option Explicit
Sub aaaaastart_fix()
Dim i As Integer
Dim doc As Document
Dim newdisp As String
Dim link
Dim r As Range
Dim oFrm As UserForm1
Set doc = ActiveDocument
For i = 1 To doc.Hyperlinks.Count
doc.Hyperlinks(i).Range.Select
Set oFrm = New UserForm1
If Left(doc.Hyperlinks(i).Address, 3) = "www" Or Left(doc.Hyperlinks(i).Address, 3) = "htt" Then
With oFrm
.TextBox3.Value = i
.TextBox4.Value = doc.Hyperlinks(i).TextToDisplay
.TextBox5.Value = ""
.TextBox5.SetFocus
.Show
End With
End If
Next i
End Sub