Hi,

go to the top right menu "administrator"  (where you can log out too), push section  "about odoo", in the opened wizard, at the top right push "activate developer mode", close the wizard.

Go to a user form, in notebook for field timezone, change the timezone value, save, change a second time value with Asia/Ho_Chi_Minh and save.

now on the top left of the form, push button "debug view", in the list choose "set defaults", in the new opened wizard, for field "default", choose in the list "Timezone = Asia/Ho_Chi_Minh", select other field "all users" and save. Now your next created user will have Timezone = Asia/Ho_Chi_Minh as default value.

see this example with pictures :

https://www.odoo.com/forum/help-1/question/default-field-value-of-a-form-view-45265

Bye