It sounds like you want something like this:
[TestMethod]
public void GetTicketId()
{
var mockMyService = new Mock<IMyService>();
mockMyService.Setup(n => n.GetTicketId(It.IsAny<Coordinate[]>))
.Returns("Not Null String");
var objectUnderTest = new ObjectUnderTest(mockMyService);
var ticketId = objectUnderTest.GetTicketId();
Assert.IsNotNull(ticketId);
}
Basically, you set up your fake item and pass it into the object that you are testing. You do not test an interface directly, you test something that is using the interface. The point of mocking is to avoid hitting external resources (or some might say even hitting more classes than you are testing..but Ian Cooper's talk is persuasive to not go down this path).