How to check a word against a dictionary from a richtextbox-VB.NET

Imports System.IO

Public Class Form1
    Dim start As Integer = 0

    Dim replacements As New Dictionary(Of String, List(Of String))
    Dim parts() As String

    Dim nextCheckIndex As Integer = 0
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        Using reader As New StreamReader("C:\Users\Acer\Desktop\Text Files1\output18.txt")
            Do Until reader.EndOfStream
                parts = reader.ReadLine().Split("|"c)


                If replacements.ContainsKey(parts(0)) Then
                    replacements(parts(0)).Add(parts(1))

                Else
                    Dim newWordList As New List(Of String)
                    newWordList.Add(parts(1))
                    replacements.Add(parts(0), newWordList)
                End If


            Loop
        End Using

        RichTextBox1.Text = "You gave a bad advice, irregardless of your intention. You provided a bad advice, irregardless of your intention. "
    End Sub
    
    Private Sub btnFind_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnFind.Click

        Dim startindex As Integer = 0

        If RichTextBox1.Text.Length > 0 Then

            startindex = FindMyText(textBox1.Text.Trim, start, RichTextBox1.Text.Length)

        End If


        ' If string was found in the RichTextBox, highlight it
        If startindex >= 0 Then
            ' Set the highlight color as red
            RichTextBox1.SelectionColor = Color.Red
            ' Find the end index. End Index = number of characters in textbox
            Dim endindex As Integer = textBox1.Text.Length
            ' Highlight the search string
            RichTextBox1.Select(startindex, endindex)
            ' mark the start position after the position of
            ' last search string
            start = startindex + endindex
        End If
    End Sub


    Private Sub RichTextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RichTextBox1.TextChanged

    End Sub


    Private Sub BtnClose_Click(sender As System.Object, e As System.EventArgs) Handles BtnClose.Click
        Me.Close()
    End Sub

    Private Sub BtnDelete_Click(sender As System.Object, e As System.EventArgs) Handles BtnDelete.Click
        textBox1.Clear()

    End Sub
End Class