To add a button in WordPress posts or pages, you can use available PlugIn. There are lots of plugin to add button but you will cannot customize follow your style or you have to pay money.
In advantage of WordPress is that easy customize, so how to add a button in WordPress posts or pages without plugin?
these are steps to add a button in WordPress posts or pages:
Step1: Login to WordPress Admin Page
Step2: Create new post.
Step3: In screen Add New Post, we will select part of Text on Editor.
Step4: At Text editor, we will add html element to create a button.
Step5: Create style for button via css