If that's in C21, you could just pick up the results of the formula:
Option Explicit
Private Sub UserForm_Initialize()
Me.TextBox1.Value =
Worksheets("sheet1").Range("C21").Text
End Sub
Or you could do the =vlookup() in code:
Option Explicit
Private Sub UserForm_Initialize()
Dim res As Variant
Dim myStr As String
res = Application.VLookup(Worksheets("sheet1").Range("A21").Value, _
Worksheets("Sheet3").Range("A2:L17"), _
Worksheets("sheet1").Range("A1").Value, _
False)
If IsError(res) Then
myStr = "Missing/no match!"
Else
myStr = res
End If
Me.TextBox1.Value = myStr
End Sub
(I'm not sure what the name of the worksheet that held c21 is.)