“Tenting” isn’t a specific option in KiCad. Vias are covered by soldermask by default, i.e. there are no round graphic items on them in the Mask layer. By default the gerbers are created accordingly. You can choose to uncover all vias when exporting the gerbers.
You don’t need to do anything special for vias placed inside pads. Pads have (should have) their solder mask layer graphics which create holes in the physical board mask, and the vias inside them are also without mask automatically. It’s not possible to cover anything with mask in KiCad in any other way than changing the visible graphics in the Mask layers. What you see is what you get, except the possibility to uncover all vias when exporting.
You may also want to read How does solder mask layer work?.
Two other tips: “tenting” option when you deal with board manufacturers may have another meaning, namely extra mask applied on vias on top of the normal mask. And “via in pad” is a special technology, a microvia which is plugged or tented with some substance so that the solder doesn’t go into the hole. A prototype board with “via in pad” will cost maybe 10x the normal cheap. You can use normal vias inside pads without worried, but they work reliably only for manual soldering. In machine assembly/soldering via holes may drain too much solder paste and make the joint unreliable.