Partial application syntax itself produces a function – it doesn’t need to be enclosed in braces.
// the expression _ + 1 produces a function
f = _ + 1 // -> a Function
f.value(2) // -> 3
// now we have a function
// *containing* the expression that produces a function
f = { _ + 1 };
// f.value evaluates the function-producing expression
// so the result is a function
f.value(2); // -> a Function
// so your g should be written
g = f.value(_, "TWO", _);
Admittedly the latter is unclear; if you find this to be unreadable (it’s a borderline case for me), then it’s always possible to write using standard arguments.
Btw please use backticks for code examples. Note in your initial post that the quotes have become typographic open/close quotes. If you copy/paste the code from this page into SC, you’ll get syntax errors for the unrecognized quotes.
Again, do not use partial application with braces: g = _ + _ should work.
The main intended use of partial application is for collect and similar methods. g = _ + _ looks weird; anArray.select(_ > 0) does not look so weird.
(Also note the accidental use of forward apostrophes for the code snip, which caused the underscores to become formatting marks – markdown for code is kind of tricky, isn’t it?)