Userform calls other userform, then populate worksheet

Try this Userform.

Private Sub UserForm_Activate()

For i = 1 To ActiveWorkbook.Sheets.Count

TN = ActiveWorkbook.Sheets(i).Name

If TN = "Main" Or TN = "Useful Code" Or TN = MyTeam Then GoTo Skip
Flag = 0

ComboBox1.AddItem ActiveWorkbook.Sheets(i).Name
ComboBox2.AddItem ActiveWorkbook.Sheets(i).Name

Skip:

Next i
End Sub

Private Sub Loader()
If ComboBox1.Value = "" Or ComboBox2.Value = "" Then Exit Sub

With Sheets(ComboBox1.Value)
On Error Resume Next
Set Z = .Columns(1).Find(ComboBox2.Value, LookIn:=xlValues, Lookat:=xlWhole)
t = Z.Row
If t > 0 Then
For Count = 1 To 4
Me.Controls("Textbox" & Count).Text = .Cells(t, Count + 1).Value
Next
End If

Adder

On Error GoTo 0
End With

End Sub

Private Sub Adder()
If Changeflag = True Then Exit Sub
For Count = 1 To 4
If Me.Controls("Textbox" & Count).Text = "" Then Me.Controls("Textbox" & Count).Text = "0"
Next

TextBox5.Value = TextBox1.Value * 1 + TextBox3.Value * 1
TextBox6.Value = TextBox2.Value * 1 + TextBox4.Value * 1
End Sub

Change this line above.

If TN = "Main" Or TN = "Useful Code" Or TN = MyTeam Then GoTo Skip

To

If TN = "Main" Or TN = "Useful Code" Then GoTo Skip

To avoid confusing yourself.

I was going to use MyTeam to create the Userform as you described it.
But then i decided that your solution was over complicated and sarted again.