Hi,

You need to inherit the web.FormView template and add the button in it.Then you can write a JS function for that button. 
Please refer to this blog to know more.https://www.cybrosys.com/blog/how-to-add-new-button-to-all-form-view-in-odoo-16

Hope it helps

You could do it using a custom widget. Check toaster_button​ as an example:

See the js code at: odoo/addons/purchase/static/src/js/purchase_toaster_button.js

Put it inside the header or where you wanna use it:

< widget name='toaster_button' button_name="send_reminder_preview" title="Preview the reminder email by sending it to yourself." attrs="{'invisible': [('id', '=', False)]}"/ >

Don't forget to define the js in the manifest of the module

'assets': {
    'web.assets_backend': [
...
        'purchase/static/src/js/purchase_toaster_button.js',
...
    ],
},