E.g. for tone 8G.

%% (c4) (g)(h) (j)(j) (k)(jr)(j) (:) (j)(j)(j)(i)(j)(h)(gr)(g.)(::)

However, it would work better with text (and is more standard, even if you don't actually use a psalm [somehow I put “small” here originally…], like in the official Antiphonale Romanum) because the spacing works best with text, and you want to show the accented syllables correctly, get the syllabification right (especially in Latin…) etc.

%% (c4) Di(g)xit(h) Do(j)mi(j)nus(j) Do(j)mi(j)no(j) me(k///jr)o:(j) (:) Se(j)de(j) a(j) dex(i)tris(j) me(h///gr)is.(g.) (::)

or %%
(c4) The(g) Lord(h) said (j) to(j) my(k//jr) Lord:(j) (:) sit(j) at (i) my (j) right(h) hand.(grg.) (::)

(Also, I belatedly realized that you're using a different site. The Source and Summit editor, the 2.0 edition of the "Alpha" editor" works with the way that I've showed you. I don't think that it works on the GABC transcription tool, which doesn't allow for anything but the most basic tasks, and even then, it needs a lot of improvement.)

@SMire it can be done on the GABC Transcription tool using Matthew's method above, you just need to select "Show integrated GABC" and encode it as he describes.

If you are interested in doing Vespers or another hour of the Office in the future (which you are, I think), I do recommend using Gregorio on your computer or even this online editor, because it becomes easier to control the position of the flex needed for longer psalm verses and, for antiphons printed separately from the psalm tone, the EUOUAE from "Saeculorum. Amen."