Here's a workaround. You can create an automated action that triggers on creation and update. The action should execute Python code. Then in the code, just write what you would write in a constrains method. Something like:

for record in records:
  if record.type == 'service' and record.purchase_method != 'purchase':
    raise Warning("Nice try, but nope!")

Not possible.. To add constraint you need some python code and odoo studio help you only to design your xml. So with the use of Studio you can't add constraint..