Hello Jan, 

Add code in your js file 

/**window.alert("Page is Reload");**/
odoo.define('Module.js file name', function (require) {
"use strict";
var form_widget = require('web.form_widgets');
var core = require('web.core');
var _t = core._t;
var QWeb = core.qweb;
form_widget.WidgetButton.include({
    on_click: function() {
         if(this.node.attrs.custom === "click"){
            //code //
         return;
         }
         this._super();
    },
});
});

and define into your xml file : 

<button name="java_script" string="Java Script" type="object" custom="click"/>

There java_script is my function name which one define in .py file.

I hope it's help you

Thanks