Deutsch Als Fremdsprache

From what I've gathered, es is often reduced to 's and gets phonologically attached to the verb in spoken German in simple indicative and imperative sentences ("wie geht's", "ich versuch's", "tu's nicht" etc).

This kind of reduction exits in other languages too, but it's blocked in some conditions. For example, in English you can say "I'm done" but not "who do you think I'm?".

What are the restrictions on the German es?

Question based on me texting someone and having to write "Er .... und hasste's wenn ich... ."

I later went to check my grammar a bit, and I couldn't find "hasste's" used anywhere on the internet. If this formulation is wrong, what's the reason why this reduction can't be used?