Thanks. Putting \placeregister[index] inside a  \startchapter[title=\mytitle{}]-\stopchapter pair does the ttrick.

> On 19 Jul 2018, at 14:19, Rik Kabel <context@rik.users.panix.com> wrote:
> 
> 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.

dr. Hans van der Meer