I see that you ignore my advice from your thread Passing value between forms. You're just making it more complicated for yourself...
It's quite possible that what you're trying to do can be done without any code, just by ensuring that you have a relation between your tables and using a menu item. You should have explained your business requirement, instead of getting into a particular technical solution, if you're not experienced enough to choose it correctly.
Anyway, back to your code:
Please learn how to use debugger - it's an extremely important tool for finding bugs in code. For example, it's possible that MyTableParent_MyID.text() doesn't return anything, therefore no subsequent code can do what you expect. Debugging will tell you whether it's the case.
If MyTableParent_MyID is bound to a table field (e.g. MyTableParent.MyID), you can throw away the whole code from form A. Use a menu item, set its DataSource property to the right datasource (MyTableParent) and get the record from element.args().record() in form B.
MyParentTable parentTable = element.args().record(); myTableChild.MyID = parentTable.MyID;