if I well understand your requierment you try to fix a start date in date range picker object. You need to fix it at first of april in the meaning of having a YTD calculation from april to current month.
You can't fixed in date range picker the start date for your date range.
Nethertheless you can see in this app that you have some predifined date range ( last 7 days , last month etc ) those predefined date range can be change in the code of the app and renamed as well.
So that, you can change the name of a predifined range as April YTD , and change the parameter to fix the start date at first of April.
See this link for parameters :
hope it helps