projectId: id of the project where you want to create project. It is displayed in the url. For instance for OneDev project, it is 160. You can also get it programmatically this way:

https://code.onedev.io/help/api/io.onedev.server.rest.ProjectResource/queryBasicInfo

milestoneIds: list of ids of milestones. Milestone ids can be queried this way:

https://code.onedev.io/help/api/io.onedev.server.rest.ProjectResource/queryMilestones

fields: list of field name and value, for instance for this issue:

"Type": "Support Request",
"Priority": "Minor",
"Assignee", "1" 

For user type field, user id is expected, and it can be queried this way:

https://code.onedev.io/help/api/io.onedev.server.rest.UserResource/queryBasicInfo

Thank you!