When using the Add button on the calendar view introduced with
odoo/odoo#64948 the event would not be linked with the applicant.

After further investigation the method used to get the applicant id in
default_get was not flexible enough, the one from crm calendar has been
'copied'

Task ID: 2578165

closes odoo/odoo#72731

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>