Bethesda Game Studios is expanding the private beta for Fallout 4's Creation Kit, the official mod support toolkit coming to the game sometime in the next month or two.
Information on how to register for the invitation-only test is here. Candidates must also sign up for the DLC beta, which includes a non-disclosure agreement covering all the content they use and the feedback they give. The closed beta is for Windows PC only.
The Creation Kit will be the last of five substantial pieces of post-release support coming to Fallout 4 over a three-month span. The first two are the premium DLC extensions Automatron and Wasteland Workshop, currently available now. Far Harbor, a full story expansion, is due sometime over the next month, and so is "Survival Mode," a free title update that ramps up the difficulty of a playthrough considerably.
The Creation Kit will follow sometime after all of this, first for PC, then for Xbox One and PlayStation 4.