How do I make an additional payment other than my direct debit
Go to your account then payments & statements and you will find a link to top up payments.
The link you mention no longer exists.
The option to pay extra used to be on there but recently disappeared from the app.
I don’t generally use the Bulb App to carry out changes to my Bulb account and when I responded I only looked at my web base Bulb account. I’ve just looked at the App and as you say there is no place to do a top up.
This Bulb article https://help.bulb.co.uk/hc/en-us/articles/360029064412-Managing-your-energy-with-the-Bulb-app mentions that at present the App is missing some useful features one being “top up your balance”.