In the bottom left outline area, click on XPage or Custom Control.
In the properties click on Resources.
Click "Add" Metadata, then click wahtever you want.
Add property name eg "viewport"
then
Content
"width=350; initial-scale=1.0; maximum-scale=1.0; user-scalable=0;"
then save and then view the xpage in the browser an the xpage source will say:
<meta name="viewport" content="width=350; initial-scale=1.0; maximum-scale=1.0; user-scalable=0;" http-equiv="viewport">