Hi,

To get selected records in the button function, we can define the tree view as follows,

    <record id="student_student_view_tree" model="ir.ui.view">
<field name="name">student.student.view.tree</field>
<field name="model">student.student</field>
        <field name="arch" type="xml">
            <tree string="Student" edit="1">
                <header>
                    <button type="object" string="Click me"
                            name="action_click_me"/>
                </header>
                <field name='roll_number'/>
                <field name='number'/>
                <field name="name" decoration-bf="1"/>
                <field name="age"/>
                <field name="email"/>
                <field name='teacher'/>
                <field name="grade"/>
                <field name='state'/>
            </tree>
        </field>
    </record>

Or we can inherit any treeview of the model and add the button in the header.

Here the button function action_click_me, we get the selected records in self.

We can get the selected records in tree view in (self._context.get('active_ids', []))

Hope it helps,

Normally we do like this

https://github.com/odoo/odoo/blob/e4f2adcf51a990e21bd113478b8cf9849c4f58b6/addons/portal/wizard/portal_wizard.py#L24-L33