Sharepoint - Add attachment button to a custom new form SharePoint 2013
Create a custom list form or go to your NewForm.aspx using SharePoint Designer. Place the below code where you would like to insert the Attachment Field
<tr>
<td width="190px" valign="top" class="ms-formlabel">
<h3 class="ms-standardheader">
<nobr>Attach Files</nobr>
</h3>
</td>
<td valign="top" class="ms-formbody" id="attachmentsOnClient" style="width: 434px">
<span dir="ltr">
<input type="file" name="fileupload0" id="onetidIOFile" size="56" title="Name"> </input>
</span>
</td>
<td width="100px" valign="top" class="ms-formbody">
<input name="Button1" type="button" value="Attach" onclick='OkAttach()' style="width: 6em;
height: 1.7em" />
<span id="idSpace"></span>
</td>
</tr>
Reference