Hi,
Do you use pure xforms or Orbeon form builder?
In the first case go to the xf:input definition, add xf:hint with appearance="minimal" attribute:
<xf:input id="control-1-control" bind="control-1-bind">
<xf:label ref="$form-resources/control-1/label"/>
<xf:hint appearance="minimal">My placeholder</xf:hint>
</xf:input>
In the second case go to the form configuration
And until now all your hints will be displayed as a placeholder (following the doc: This is only supported for text, date, and time input fields.)
//Jarek