I know this is out of context. But since this is top result when searching this case in google, I would like to add an answer if anyone want to do it in Odoo 16

    <record id="action_button" model="ir.actions.server">

        <field name="name">Action Button</field>

        <field name="type">ir.actions.server</field>

        <field name="model_id" ref="model_sale_order" />

        <field name="binding_model_id" ref="model_sale_order" />

        <field name="state">code</field>

        <field name="code">action = python_func()</field>

        <field name="binding_view_types">form</field>

    </record>

Add <field name="binding_view_types">form</field> in the record. You can set it to 'tree,form' if you want to have it in both or remove field binding_view_types if you want the button appear in all view of the same model

I wrote a module 'web_menu_action_if' to accomplish this task in Odoo 9.0.
See my answer to:

https://www.odoo.com/forum/help-1/question/assign-action-menu-item-to-specific-tree-view-112007

Hello Marek Jan-Alexander,

You can add through Settings > Technical > Server Actions in that select your action and in form view top right corner there is a button Add in the "More" menu.