You are gonna need to use an iterator to check all the forms in the website. In this case we will be using for
. But this doesn't let us know about which form we are working on, it just lets us use it. So we are going to assign 0(the first form's ID) to a variable and add 1 to it as we change forms when a new iteration/loop starts.
currentForm = 0
for form in br.forms(): # For every form in the website
currentForm += 1 # Add 1 to the current form so that the script knows which form we will be working next
if not forms.find_control(name = "birthday"): # If the form isn't about birthday
continue # Go to the next iteration / loop ignoring the statements below
br.select_form(nr = currentForm) # Select the birthday form
br.form["birthday"] = "Fill this with what you want" # Writing to the form
br.submit() # Submit the working form
Note: x += y
is equal to x = x + y