the site says it's a 3/4" slip so you may need to get a 3/4" coupler and use it to mate a 3/4" slip barb fitting. I'd take the pump fitting to home depot and check to see if it slips on.
You are better off getting a 1.25" thread/slip union, then you can go either hard pipe or barb. Hardpipe is the way to go. Barb restrict flow at the tip. GL
I have to see the actual product before making my recommendation on this one.
From my experience with this, you have to be very careful with the sizes, otherwise it will not work.