How to make Black Glutinous Rice sticky?
I don't know what is done traditionally but have you seen this recipe? They use part white sticky rice as the black rice alone did not seem sticky enough. 
Mango with Black Sticky Rice | Khao Niew Dam Mamuang

If you are interested there is also a way to make it more like a porridge that is uses more water and is cooked longer like congee in a pot and becomes somewhat glutinous. Thai Black Sticky Rice Pudding I'm not sure the eggs are necessary, I've never heard of that in the Indonesian version.