'ブックB
Sub Sample()
Dim p As String
Dim wb As Object

p = "C:\Users\user\Desktop\A.xlsm"
Set wb = Workbooks.Open(p)

wb.VBProject.VBComponents("UserForm1").Designer.Controls("textbox1").Value = "テスト"
MsgBox wb.VBProject.VBComponents("UserForm1").Designer.Controls("CommandButton1").Caption

Application.Run "A.xlsm!フォームを開く"
Application.Run "A.xlsm!コマンドボタンクリック"
End Sub

'-----------------------------------------------------------
'ブックA
Sub コマンドボタンクリック()
Call UserFormA.CommandButton1_Click
End Sub

Sub CommandButton1_Click()
MsgBox "メッセージ"
End Sub