Here is an example found. You just have to use the right idMso tab in the custom backstage UI



<tab idMso="TabRecent">
<firstColumn>
<taskFormGroup idMso="GroupOpenPlaces">
<category idMso="ButtonCategoryRecentDocuments">
<task id="myTaskFirst" insertAfterMso="ButtonTaskRecentDocuments" label="Custom Task">
<group id="myGroup" label="Custom functionality" helperText="This group contains custom functionality.">
<primaryItem>
<button id="myButton" label="My Button" onAction="CallMe"/>
</primaryItem>
</group>
</task>
</category>
</taskFormGroup>
</firstColumn>
</tab>