To create a new Click-to-Messenger campaign, follow the steps in the Facebook Ads Manager campaign creation process until you come to the step where you select your Marketing Objectives.
Select "Conversions" as your marketing objective. Set up the campaign settings as you normally would.
After you've created your Conversions campaign, you will need an adset with Messenger conversions, not website, selected. We've set our campaign here to optimize for Purchase events.
Set up the targeting settings as you normally would.
After you have completed your adset targeting, set up your ad as you normally would until you get to the bottom card, titled Messenger Setup.
Make sure you have selected the "Create your own welcome experience" button and have the "Custom" tab selected. Click the "+ Create Message Button".
A modal will appear. Select the JSON tab. You can also name your message template at this time.
An input field will appear that has been prefilled with Facebook's default message template. Select All (Command/Ctrl + A) and delete everything.
It's important to paste your own JSON into the top-level, so make sure everything is deleted first.
Paste your JSON into the input field. It will format itself. Click "Finish" to complete the ad. A dialog box will ask you to confirm your message so it can be saved. This is a good time to do a last preview of your message and confirm that your message name is descriptive enough to stand out in a list.
After your custom JSON message template has been created, it will appear in your list of existing templates, under the Existing tab. Click "Confirm" to create your ad!