opentype

You could just add it to the menu as link. Something like: https://invisioncommunity.com/forums/startTopic/
If it really should be a clearly visible button, you would create a custom Pages block in the ACP with the link styled as primary button. Something like this:
<div class="i-padding_1"> <a class="ipsButton--wide ipsButton ipsButton--primary" href="/forums/startTopic/">New Topic</a> </div>This is a code example for a sidebar button in 5.x I use.