Similar to https://jira.atlassian.com/browse/JRACLOUD-71153 but this API is for creating and updating issues.

The Jira REST API for creating and updating issues should allow you to assign an issue to the default assignee by specifying "-1", according to the documentation.

e.g.

HTTP POST to /rest/api/2/issue

This works when specifying the assignee by name:

{
	"fields": {
		"assignee": {
			"name": "-1"
		},
		"project": {
			"key": "TES"
		},
		"issuetype": {
			"id": "10004"
		},
		"summary": "hello"
	}

}

but not when specifying the assignee by account ID.

{
	"fields": {
		"assignee": {
			"id": "-1"
		},
		"project": {
			"key": "TES"
		},
		"issuetype": {
			"id": "10004"
		},
		"summary": "hello"
	}

}