Prime unfortunately is page oriented and the dotted line indicates the right margin of the pages when you are in draft view. Expressions to the right of it are considered in calculations but unlike in Mathcad cannot be printed, so the dotted line tells you what you will see in the printout (and what not).
The only way to "move" that line to the right is to set a wider page. You can do this by choosing narrower margins and a larger paper page size. Unfortunately the latter is very limited, compared to real Mathcad where all paper sizes the current printer driver provides could be chosen (by choosing an appropriate pdf printer, paper size could be set to any possible width, virtually unlimited). In Prime you are limited to a few predefined sizes.
The largest width can be achieved in Prime by choosing paper size "Ledger/Tabloid" in "Landscape" mode, followed by "A3" in "landscape" and choosing 0,25" (the smallest value which will be accepted) for the left and right margin.
But of course doing so will get you into troubles when you print to a physical printer with smaller paper 😉
