Here is your code, slightly rearranged to ask the user for input. If he enters nothing "???" the code terminates. If he enters something else that string is incorporated into your FIND function and that function string printed to the Immediate pane.
Sub Check_Parent()
Dim strFormula As String
Dim Result As String
Dim lR As Long
Result = InputBox("What to look for?", "Search criterium", "???")
If (Len(Result) = 0) Or (Result = "???") Then Exit Sub
strFormula = "=IFERROR(LEFT(RC[4],FIND(""" & Result & """,RC[4])-1),RC[4])"
Debug.Print strFormula
' addressing the 'Activesheet' usually is a bad a idea
' and it's a bad idea here, too.
' use code like Worksheets("Sheet1") to name the sheet or 'Worksheets(1) '
With ActiveSheet
lR = .Cells(.Rows.Count, "A").End(xlUp).Row
.Range(.Cells(2, "B"), .Cells(lR, "B")).ClearContents
End With
' Worksheets(1).Columns("B").EntireColumn.Insert
' Worksheets(1).Columns("C").EntireColumn.Insert
' Worksheets(1).Range("B1").Formula = "Value 1"
' Worksheets(1).Range("C1").Formula = "Value 2"
' lr = Range("A" & Rows.Count).End(xlUp).Row
' 'ActiveSheet.Range("B2:B" & lr).Clear
' Range("B2").Select
' Columns("B:B").Select
' Selection.Copy
' Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
' Selection.TextToColumns Destination:=Range("B1"), DataType:=xlDelimited, _
' TextQualifier:=xlDoubleQuote, ConsecutiveDelimiter:=False, Tab:=True, _
' Semicolon:=False, Comma:=False, Space:=False, Other:=False, FieldInfo:=Array(1, 1), _
' TrailingMinusNumbers:=True
' ActiveSheet.Range("C2:C" & lr).Select
' Selection.FormulaR1C1 = "=IF(RC[-2]=RC[-1],TRUE)"
' Columns("C").Select
' Selection.Copy
' Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
End Sub
Mixing the ways to address sheets is not good. If Worksheets(1) is the ActiveSheet then it should be addressed as Worksheets(1) because addressing the ActiveSheet is dangerous except in a very controlled circumstances. Workshets(1) is the first tab on the left. The user can change that, too, causing your code to create chaos. Therefore it's best to use sheet namesd in the code.
The rest of your code I neither checked nor examined. However, I saw that you have several Select statements in it. They should be removed. VBA knows where all the parts of the workbook are. Therefore you should use the Range object instead of the Selection object. Define a range by specifying its first and last cells as I have shown you. Then you can do with it whatever you want. Note that code like Range("B2:B" & lR) does indeed specify a range, and Range(Cells(2, "B"), Cells(lR, 2)) does look longer but the latter is more efficient despite of that because Excel uses numbers to identify rows and columns and the range address you take so much pain to create a string of must be converted back to numbers for Excel's use.