To do that you need to add this code in a loaded js file of your module defined in the backend assets template(search others posts for that), changing the code in bold for your needs

instance.web.form.AbstractFormPopup.include({
    setup_form_view: function() {
        var self = this;
        if (this.row_id) {
            this.dataset.ids = [this.row_id];
            this.dataset.index = 0;
        } else {
            this.dataset.index = null;
        }
        var options = _.clone(self.options.form_view_options) || {};
        if (this.row_id !== null) {
            options.initial_mode = this.options.readonly ? "view" : "edit";
        }
        _.extend(options, {
            $buttons: this.$buttonpane,
        });
        this.view_form = new instance.web.FormView(this, this.dataset, this.options.view_id || false, options);
        if (this.options.alternative_form_view) {
            this.view_form.set_embedded_view(this.options.alternative_form_view);
        }
        this.view_form.appendTo(this.$el.find(".oe_popup_form"));
        this.view_form.on("form_view_loaded", self, function() {
            var multi_select = self.row_id === null && ! self.options.disable_multiple_selection;
            self.$buttonpane.html(QWeb.render("AbstractFormPopup.buttons", {
                multi_select: multi_select,
                readonly: self.row_id !== null && self.options.readonly,
            }));
            var $snbutton = self.$buttonpane.find(".oe_abstractformpopup-form-save-new");
            $snbutton.click(function() {
                alert('test');
                $.when(self.view_form.save()).done(function() {
                    self.view_form.reload_mutex.exec(function() {
                        self.view_form.on_button_new();
                    });
                });
            });
            var $sbutton = self.$buttonpane.find(".oe_abstractformpopup-form-save");
            $sbutton.click(function() {
                $.when(self.view_form.save()).done(function() {
                    self.view_form.reload_mutex.exec(function() {
                        self.check_exit();
                    });
                });
            });
            var $cbutton = self.$buttonpane.find(".oe_abstractformpopup-form-close");
            $cbutton.click(function() {
                self.view_form.trigger('on_button_cancel');
                self.check_exit();
            });
            self.view_form.do_show();
        });
    }
});