Adding Foreign Keys to Custom Table from default modx parent table

I have installed modx revolution. I have also added few new custom tables. I have my new 'registration' table with a column 'userid'. I want to add this as foreign key(with n delete and on update cascade) linked with 'id' from modx_user default table.
How can I do that? I have created my tables with mysql CREATE queries.
When I add foreign key from default tables the error message is 'can not add foreign key'

what you want, is extending the modUser
https://rtfm.modx.com/revolution/2.x/developing-in-modx/advanced-development/extending-moduser

you might also want to have a look to
http://bobsguides.com/classextender-class.html

Quote from: Bruno17 at Feb 04, 2016, 04:57 AM

what you want, is extending the modUser
https://rtfm.modx.com/revolution/2.x/developing-in-modx/advanced-development/extending-moduser

you might also want to have a look to
http://bobsguides.com/classextender-class.html

Can you give me an example according to my question.
I have registration table. Columns in it --> (reg_id, userid, fullname,email)
Link userid as a foreign key to id in modx_users table.
I have tried XML schema <aggregate ....>

Also provide me an example on how to insert data into two tables at a time provided both tables are linked with foreign key. Example when we create a new user, the data is inserted into modx_users and modx_user_attributes. How is it happening? Insert data into two tables at a time.....!!

examples?
this is the processor for updating the user and the user-profile:
https://github.com/modxcms/revolution/blob/2.x/core/model/modx/processors/security/user/update.class.php#L128

this is Bob's plugin for his extented table
https://github.com/BobRay/ClassExtender/blob/master/core/components/classextender/elements/plugins/extrauserfields.plugin.php#L44