How to trigger system form functions from ISV buttons

• open up the form in question
• hit F12 to open IE Developer Tools
• Find button in question
• Find the function name
• Export ISV.config
• Within relevant entity add a new button and paste function into Javascript tag
  
• See below code to create an associated record from ISV of parent form
• (in this case it's a custom entity called Site Survey Request from Opportunity entity)

{Entity name="opportunity"}
{ToolBar ValidForCreate="0" ValidForUpdate="1"}
{Button JavaScript="locAddRelatedToNonForm(10045,3,crmForm.ObjectId, '')" ValidForCreate="0" ValidForUpdate="1"}
{Titles}
{Title LCID="1033" Text="Request Site Survey" /}
{/Titles}
{/Button}
{/ToolBar}
{/Entity}