This is the macro I use to open the form:

Sub oDoc_1( )
Dim ObjTypeWhat
Dim ObjName As String
ObjTypeWhat = com.sun.star.sdb.application.DatabaseObject.FORM
ObjName = "My.Home.Form" 'The name of the form you want to open at Base start-up, this needs to exist.
If ThisDatabaseDocument.FormDocuments.hasbyname(ObjName) Then 'Check the form exists
ThisDataBaseDocument.CurrentController.Connect() 'If the form exists connect to the database
ThisDatabaseDocument.CurrentController.loadComponent(ObjTypeWhat, ObjName, FALSE) 'Open the form
Else
MsgBox "This is embarrassing, sorry, can't find the requested form to open!"+chr(10)+"Form Name = " & ObjName + chr(10)+"Check the details." , 48, "DS4A SBM encountered a problem!"
End if
End Sub

It works properly on Linux, but on OSX, the form opens, but displays invisibly.  It sometimes flashes visible on closing LO, which isn't useful.