Line spacing in text block and text box

Add 1pt line for spacing.

I guess you are talking about Prime, correct?

Its far away from being a word processor and misses a lot of features in that respect - being able to set the line spacing is one in a very long list of non-existent features. Incomprehensibly, we don't even have superscripts and subscripts in text fields - and that with software that is aimed at engineers and should also support them in documenting their work!
However, the missing features in the actual range of functions (compared to the more powerful over 10-year-old real Mathcad) are much more serious and painful than the missing word processor features (judging in this respect may depend on your needs, of course).

There may be more or less useful workarounds for the failures of PTC, but it is still incomprehensible that even simple and certainly easy to implement features have not been upgraded up to version 6 of Prime and I don't have much hope for the upcoming version 7.

The workaround suggested by ttokoro (manually adding blank lines with a very tiny font size to simulate larger line spacing) will also mean that you cannot use the automatic line break feature any more, which may be a problem if you later decide to resize a text box.