Hello @Marin Čović,
I'm afraid that there is little customization about SFI_SUBMIT_BUTTON in Custom credential providers.
You can open an incident at https://developer.microsoft.com/en-us/windows/support/?tabs=Contact-us and please choose the 'Security Development -> Credential provider API' for Windows SDK for this issue. Or submit an issue at https://github.com/microsoft/Windows-classic-samples/issues for possible response.