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After you exit the game, click the Sims 4 icon to relaunch the game. If you haven't already installed the Tmex-AllCheats mod, see Method 1 to learn how to install the cheat mod.  You can only install mods on the PC version of The Sims 4. After the title screen animation, click Load Game and click the game you want to load.  Click the Play triangle icon in the lower-right corner to load the game. Select the household with the Sims you want to marry and play as one of the Sims. This opens the command console.  You can use this to enter commands and cheats in the Sims 4. This enables cheats in your game. Replace [Sim's first name] ad [Sim's last name] with the first and last name of the Sim you want to marry.  This displays a number for the Sim in the console. The Sims need to be Young Adults or older. Use the "sims.get_sim_id_by_name" to the Sim ID for the second Sim you want to marry.  Both Sim ID numbers should be displayed in the console.  You don't need to be playing as that Sim to get their ID as long as you type the correct first and last name. Replace [Sim A's ID] with the numeric ID of the first Sim you want to marry, and replace [Sim B's ID] with the numeric ID of the second Sim you want to marry.  Both IDs should still be displayed in the console.  If you are currently playing as one of the Sims you marry, it will display the "Just Married" perk in the lower-left corner. If you can't get the relationshipBit cheat or Tmex-AllCheats.ts4script to work, you can use the modifyrelationship cheat to raise the relationship between the Sims so that you can manually make them propose.  Type modifyrelationship [your Sim's full name] [other Sim's full name] 100 friendship_main into the console and press ↵ Enter. This raises their Friendship status is necessary, as the proposal will be rejected otherwise.  Type in modifyrelationship [your Sim's full name] [other Sim's full name] 100 romantic_main into the console and press ↵ Enter. This raises their Romantic relationship, enabling them to start dating.  Once their romantic relationship status is high enough, "Propose" will be available under "Romantic" when you click on the Sim you want to marry.

Summary:
Open the Sims 4. Load the game with Sims you want to marry. Select the first Sim you want to marry. Press Ctrl+⇧ Shift+C. Type in testingcheats true and press ↵ Enter. Type sims.get_sim_id_by_name [Sim's first name] [Sim's last name] and press ↵ Enter. Repeat for the second Sim you want to marry. Type relationship.add_Bit [Sim A's ID] [Sim B's ID] romantic-Married and press ↵ Enter. Try the modifyrelationship cheat instead.