The redirect function takes keyword parameters in the function call.
Off-the-cuff - I’m sure my syntax is wrong on one or more items here:
in a urls.py in your urlpatterns
path('transactions/<int:portfolio_id>/', views.add_transactions, name='add-transactions'),
your view can take the portfolio_id as a parameter:
def add_transactions(request, portfolio_id):
...
which means you want to:
return redirect('add-transactions', portfolo_id=portfolio_id_from_form_or_wherever)