The claim is made once, in a single stanza of Grimnismal, and it is startling in its brevity: Folkvangr is the ninth, and there Freyja decides the seating in the hall, choosing half of the slain each day while Odin has the other half. Her hall is named Sessrumnir, the roomy-seated. That is very nearly all the corpus says.
Everything a reader wants to know is absent. Whether Freyja chooses first, whether her half are chosen by different criteria, what they do there, and whether they fight at the end as Odin's do, are all unanswered. Later prose adds little. The silence is conspicuous because Valhalla is described at length, repeatedly, with its doors and its boar and its daily fighting.
Scholars have proposed that the two halls represent competing traditions imperfectly merged, or that Freyja's claim is connected to her role in receiving the dead more generally, and neither can be established. Some have suggested the ship-shaped stone settings found in Scandinavian burial fields relate to the name Sessrumnir, which can be read as a term for a ship, but this remains a proposal.
The entry is here as it stands in the sources, which is a strong claim with nothing built on it. Filling the gap would mean inventing, and the gap is itself informative about how much of this material has been lost.