1 make a static DataParameter<Boolean> field in your Events class
2use ObfuscationReflectionHelper.getPrivateValue inside a static block https://www.geeksforgeeks.org/g-fact-79/ to init the field
the fieldname is "field_184714_b"
3 in your event handler method check if the entity is a creeper
4 get the EntityDataManager and set the value with the DataParameter field in the class