causes validation not working

Question

hi

 

for validate in asp.net code below:

 

  <input type="text" id="key_post" runat="server" style="width:300px" placeholder=" " maxlength="200" required oninvalid="this.setCustomValidity('key post')" onchange="this.setCustomValidity('')" />

 

and button causes validation not work 

<button type="submit" class="btn btn-cream" runat="server" id="Button1" onserverclick="btn_Send_Post1" causesvalidation="false">exit</button>