Beat worth (?)

Hello, I need help figuring out how to make one beat equal to a double dotted half note (if that makes sense). I like recreating classical pieces in Musescore and like to make them as close to the original, but I don't know quite how to do this or even where to begin. I don't know anything about this piece so if someone knows, please tell me what the line above means (and it's not an 8va bc there is one of those later in the piece), also the composer wasn't kind enough to give any form of time signatures on his pieces. The image below shows what I am referring to.

TL;DR: How do you make a singular beat equivalent to a double dotted half note