You should add smsapi-counter.js or smsapi-counter.min.js files at the end of <body> element. You can find an example in demo.html file.
Activate counter with specified attributes. You have to target <textarea> and one random element (span, h1, h2 etc) to show current characters count. With HTML you can activate one counter per page. To include more text areas with counters, use JavaScript activation method.
<textarea data-cv-sms-textarea></textarea>
<span data-cv-sms-counter></span>After loading smsapi-counter JavaScript file, use this method. You have to specify two parameters: ID of <textarea>, and ID of label that will show current characters count.
CVSMSCounter.activate('textareaID', 'counterLabelID');We reccomend to use UTF-8 encoding. Script might count characters wrong if you will use wrong encoding or message is filled using copy/paste method. If you want to protect additionally you can use parameters like nounicode, normalize, max_parts. You can find more details about parameters in SMSAPI.com specification.