The donate on behalf of feature was removed because it was causing some issues. When someone donated for someone else and then charged back, the user who they donated for was getting banned. That and sometimes people don't want a donation made on their behalf. Not many people were using the feature and to fix/change the back-end to handle things properly would have been more work than it seems to be worth...
Sorry if this is an inconvenience, but it helps us keep donations reliable and safe when we know exactly who donated what, as opposed to the on behalf of transactions which are difficult to manage.