If you want to add YouTube Subscription Button to your WordPress site you can use the following reference from Developer's section on Google.
https://developers.google.com/youtube/subscribe/reference

It's a matter of copy/pasting an HTML snippet of code.

The only small detail that you need to pay attention to is these two attributes.
data-channel and data-channelid.

data-channel <= that's your channel attribute
data-channelid <= that's your channel ID

Get YouTube subscribe button with your channel username

If you have claimed a username for your channel you can use data-channel attribute.

<br/>
Subscribe to our YouTube channel 
<script src="https://apis.google.com/js/platform.js"></script>
<div class="g-ytsubscribe" data-channel="YOUR_CHANNEL_USERNAME"></div>

Get YouTube subscribe button with your channel ID

If you don't have a username for your channel you can use data-channelid attribute.
Here you need to specify the channel ID.

<br/>
Subscribe to our YouTube channel 
<script src="https://apis.google.com/js/platform.js"></script>
<div class="g-ytsubscribe" data-channelid="YOUR_CHANNEL_ID"></div>


Here is how to get your channel ID

How to get to your channel id