On 7/19/2018 07:54, Hans van der Meer wrote:
> The macro \completeregister[index] placed in 
> \startbackmatter\startappendices typesets as “I Index”.
> How can I change and format with my custom macro the part “Index”?
> Such as using \mytitle{xyz} instead of Index?
>
> No solution found in the Context Wiki.
>
> dr. Hans van der Meer
>

Try \placeindex or \placeregister[index] and your own section heading. 
This is analagous to \completecontent and \placecontent.