CreateBox has been replaced by its successor, InputBox. InputBox is a more generic input form extension that provides more options than CreateBox does. Both are enabled on legacy wikis but only InputBox is enabled on UCP wikis. You can view Help:InputBox and mw:Extension:InputBox for detailed information on the extension.
In terms of converting createbox tags into inputbox tags, it should be as simple as changing the tag name and adding the line "type=create". As discovered in this thread, there may be some slight differences in terms of default values. For example, the default box widths are different between the two extensions. However, most of those differences can be remedied by specifying the value of the applicable parameter. In the case of the box width, adding the line "width=20".
Also please note that preloaded parameters may not work with the UCP visual editing mode. This issue was raised here but, to my knowledge, there has been no statement from Fandom on the issue.