How can I start a Job by X++ Code?

You can do that, but using a job instead of a class has only disadvantages (e.g. jobs aren't able to run on server side). Don't use jobs in any production code.

In general, you have two possibilities - to create a menu item and call it:

//you can change the type of called object without changing the invocation code
MenuFunction::runClient(menuItemActionStr(YourMenuItem), MenuItemType::Action);

or to run an application object via TreeNode:

#AOT
;
TreeNode::findNode(strFmt(#JobPath, identifierStr(YourJob))).AOTrun();