Hi,
Sorry for the late reply. You can do this by using the developer tools to inspect the “New child” button. Look at the “spjs.vLookup.addChild” function call. Copy it and create a custom button in a HTML section like this – see how I have appended the ContentTypeId to the URL:
<input type="button" value="New child item with CT" onclick="spjs.vLookup.addChild('/DFFS/Lists/DFFS_ContentTypes/NewForm.aspx','parentListGuid={c5c44b98-34f1-4ade-87d3-ed292eee0d84}&parentForm=/DFFS/Lists/DFFS_TestList/DispForm.aspx&parentItemId={ID}&baseUrl=/DFFS&getFromParent=Title|Title|false&ContentTypeId=0x011100414D8791294AE9428E4C2DEE31828919','vLookup_dummy',false,false);return false;">
Please note that I have used {ID} in the “parentItemId” parameter in the function call to pull the current items ID.
You can create multiple buttons like this to add different content types.
Let me know how this works out.
Alexander