Introduce a new Dynamic Placeholder Generator in mass mailing and SMS creation. Using a specific character (#) in subject or via the PowerBox in body we open a field selector popup, which can be use to select the wanted field and enter a default value. on validation, the dynamic placeholder will be inserted in the corresponding input/editable. task-2154624 closes odoo/odoo#71878 Related: odoo/enterprise#30333 Related: odoo/upgrade#3766 Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>