Fixing specific fields that fail to autofill on specific websites

I have noticed that specific websites sometimes fail to autofill specific fields from my 1Password entries. I presume this is because the website developers used non-standard markup, and so 1Password cannot recognize the field properly.

If one were comfortable with Chrome Inspector, and sufficiently motivated to solve the issue, is there a guide or document that describes the steps involved?

———

👇 A specific example: the https://send.royalmail.com/ payment form fails to fill the CVV field from my payment entries.

Inspecting the input element, this is the markup:

<input id="securityCode" name="securityCode" type="text" inputmode="numeric" maxlength="3" data-minlength="3" placeholder="" data-length-message="Enter a valid security code" data-mandatory-message="Enter a valid security code" class="textbox pin security-code" ontouchstart="" data-populate="card-security-code" value="" aria-describedby="pin-helptxt securityCode-hint" autocomplete="cc-csc">

The default name of the field in my 1P entry is verification number. I tried manually creating three new fields (of type "Password") with names: cc-csc, card-security-code, and securityCode, but to no avail.

1Password Version: 8.10.4
Extension Version: 2.9.0
OS Version: 13.2.1
Browser:_ Arc (Chromium)