first launches their election forecast model for the 2024 US Presidential Election, this market resolves to the percent chance the model says that Trump has to win the election. 

If 538 has multiple models, this resolves based on whichever model they present as the "main" model. In 2020 there was only one version of the model, but in 2016 there was "poll-plus", "polls-only", and "now-cast". Polls-only was the "main model", in that case.

The model launched on Jun 29th in 2016, and August 12th in 2020. But perhaps it will be earlier this year, given that the primaries are already settled. 