Get Value Without OnClick Js : i am finding solution for last 1 week but still no sucess so finally came here please help me out,

i want whatsapp share button for my android app , all is set only problem is i can get text area value but when i click ob button

<button onclick="GetValue ();">Get the velue of the textarea!</button>

i want to GetValue(); without onclick

this is my whatsapp code
<a href="whatsapp://send" data-text="GetValue ();" data-href="" class="wa_btn wa_btn_s" style="display:none">Share</a>

I want textArea Value in data-text and i am using it like above but it dont work.

this is my textarea
<textarea id="input_output" style="width:95%;" rows="10" wrap="soft">Enter Text Here..
</textarea>

please is there any way to get value without clicking button ?