How to turne on SMS passcode autofill

I'm not aware of a setting for that, I thought I remembered that it was a setting in the app itself. I did a search of all settings in the Settings app on the iPhone and could not locate anything there. With my particular bank, it asks me how to send a verification, it can be email or text. I select text. When the text arrives, I click into the area I'm supposed to enter the code, and the code from the text message appears above the keyboard, like predictive text does. I can ask if you have predictive text turned on? Or ask your friends, if they are also using an iPhone and the same banking app, and ask how they set it up. You should be able to compare settings.

Otherwise, you can contact your bank by telephone. They should have someone there to provide support for the app and I doubt you are the first person to ask this question. If they cannot answer it, then use the Get Support link at the top of the page here and contact Apple Support to ask them.

Fill in for what? It won't work for everything, unless it is coded for that. I've found it works fine for my banking app, and that is the only one that I have that uses an SMS verification code.