Customize source parameter Sharepoint 2013

I have to customize the returning from saving and exiting (by canel) a list element. I know that is done by the source parameter in the url, but i did not understand how to set/edit it. If i take a look to my sourcecode i see the follwing anchor:

<a id="idHomePageNewItem" class="ms-heroCommandLink" href="http://portal/projects/buchungsportal/_layouts/15/listform.aspx?PageType=8&amp;ListId=%7BA57192AA%2D80F8%2D4C4E%2D91DA%2D94131DCC2F47%7D&amp;RootFolder=&amp;BetriebId=1265" 
    data-viewctr="6" onclick="NewItem2(event, &quot;http://portal/projects/buchungsportal/_layouts/15/listform.aspx?PageType=8&amp;ListId=%7BA57192AA%2D80F8%2D4C4E%2D91DA%2D94131DCC2F47%7D&amp;RootFolder=&amp;BetriebId=1265&quot;); 
    return false;" target="_self" title="Ein neues Element zu dieser Liste oder Bibliothek hinzufügen.">
    <span class="ms-list-addnew-imgSpan16"><img id="idHomePageNewItem-img" 
        src="/_catalogs/theme/Themed/982A91AA/spcommon-B35BB0A9.themedpng?ctag=6" class="ms-list-addnew-img16">
    </span>
    <span>Neues Element</span>
</a>

When i click on my anchor the folloeing url opens:

http://portal/projects/buchungsportal/Lists/Anfragen/NewForm.aspx?Source=http%3A%2F%2Fportal%2Fprojects%2Fbuchungsportal%2FLists%2FBetriebe%2FAllItems%2Easpx&RootFolder=&BetriebId=1265

Where does the source parameter come from? How can i customize it before sending the request to the server?