Currently, this feature is not supported, as Jira Action items were rolled out only recently. If you review the comments on the original post, you'll notice that other Jira users like yourself have highlighted similar limitations.
To address these gaps, we've compiled a guide that compares these limitations with the features offered by a comprehensive checklist plug-in like Didit. You can find the detailed comparison here.
Hi - it looks like you're talking about the new Action Items in Jira.
You are correct that markdown is not supported for adding checkboxes ("action items") via the API, Automation, etc.
I haven't tried using the API to add tasks, but looking at the API v3 Get issue endpoint you can see how Action Items (checklist) data is stored in the Description field, with a new content type, taskList that contains and array of taskItems. Example here.
@David Bakkers found that you don't need to specify a valid localId parameter, so you should be able to start with his example here.
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