In real Mathcad this option unfortunately was limited to text and did not work, when you edited a program and added some lines there.
In Prime this again is a missing option at all.
For text you can use the text blocks, as already mentioned by Terry. Inserting math regions inside text block may have its own drawbacks, though. At least I try to avoid doing so as I found it makes editing and aligning even harder.
As an alternative you could use a collapsible area and put texts and math regions inside. Of course you must not collapse it and you also can hide the border, so all that is seen is the minus button at the far left.
Adding regions (math or text) in this area will enlarge the area and push down the regions below it.
Inside the area you will of course encounter the same problem as in the worksheet - regions are not pushed down when inserting or enlarging regions above it.
You may also combine text blocks and areas by using text blocks inside the area and of course you can insert a couple of separate areas, one below each other.
