Short answer. By a verified petition for judicial authorization. Where spouses are separated in fact or one has abandoned the other, and the consent the law requires is withheld or cannot be obtained, you may petition the court — attaching the proposed deed, or describing the transaction if there is none.
What the law says
When a husband and wife are separated in fact, or one has abandoned the other and one of them seeks judicial authorization for a transaction where the consent of the other spouse is required by law but such consent is withheld or cannot be obtained, a verified petition may be filed in court alleging the foregoing facts.
Family Code, Article 239 — Petition Where a Spouse's Consent Is Withheld. Read the full provision →
This is the lawful alternative to his signature
The law requires the written consent of the other spouse for a disposition or encumbrance of community or partnership property, and without it the transaction is void. That rule would trap a spouse whose husband has vanished, so the Code supplies the substitute: authority of the court. The article covers both variants of the problem. Consent may be withheld, where he is reachable and simply refuses, or it may be that it cannot be obtained, where he is gone. Either fact opens the petition.
The transaction must be identified, not general
You are not asking for a standing power to deal with the property. The article requires the petition to attach the proposed deed, if any, embodying the transaction, and where no deed exists yet, to describe in detail the said transaction and to state why consent cannot be secured. So a buyer, a price and terms are practically part of the application. A petition asking for leave to sell an unspecified property to an unspecified buyer at an unspecified price is asking the court to authorise something it cannot examine, which is the opposite of what the provision contemplates.
The court sees the deal twice
The last sentence is the one that catches sellers out at the closing table: In any case, the final deed duly executed by the parties shall be submitted to and approved by the court. Authorisation to transact is not the end of the court's involvement. The signed deed goes back for approval, so the terms actually executed must match what was authorised, and a deal renegotiated after the order — a reduced price, a changed buyer — has to be put before the court rather than quietly closed. Build that second step into your timetable with the buyer.
The facts to allege, and to prove
The petition is verified, so its allegations are made under oath and need to be ones you can support. Be ready to establish the separation in fact or the abandonment and when it began, the efforts made to locate him or to obtain his consent, and the character of the property as community or partnership property. Bring the title, the marriage certificate, whatever record you have of his departure and of your attempts to reach him, and the proposed deed. The court also acts on proof that notice was given to him, so his last known whereabouts matter.