This seems like a simple and very helpful feature to add. It could be a field similar to default value that can be set statically or with a formula but it would disappear as soon as it's clicked inside and would be considered blank if the field not actually filled out.

It could simply be light gray text (or even better a selectable color that has a default of light gray) that appears. It would greatly save space on building forms if you didn't have to add in space to label what ever field is intended to be and the place holder text could serve as that.

There are a lot of benefits and a common feature in many form building tools.