Specifically, you can enter these as lyrics (Ctrl+L) which I find most convenient, or chord symbols (Ctrl+K), or staff text (Ctrl+T). It is tempting to try to use figured bass (Ctrl+G) but even though the concepts are simple, it wasn't really designed for this, and sometimes the differences get in the way of things working as expected for roman numeral analysis.
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Found on the computer keyboard...
Use combinations of the letters: I, V, X, C, D, and M.
Specifically, you can enter these as lyrics (Ctrl+L) which I find most convenient, or chord symbols (Ctrl+K), or staff text (Ctrl+T). It is tempting to try to use figured bass (Ctrl+G) but even though the concepts are simple, it wasn't really designed for this, and sometimes the differences get in the way of things working as expected for roman numeral analysis.