the usage of LASSEN
hallo Leute! I have a question about the word 'lassen'.
so I think it´s similar to ´let´ in English, right? When I want to say let me know in German, then should I say lass es mich wissen? or lass mich wissen?
further, if you could tell me more about the usage of lassen, I would be really appreciate for that!
herzlichen Dank!

Question

the usage of LASSEN
hallo Leute! I have a question about the word 'lassen'.
so I think it´s similar to ´let´ in English, right? When I want to say let me know in German, then should I say lass es mich wissen? or lass mich wissen? 
further, if you could tell me more about the usage of lassen, I would be really appreciate for that!
herzlichen Dank!

Answer

Lassen is the German cognate of the English verb to let. But it is not always wise to translate phrases word-by-word, even when languages are related. A better translation of "let me know" is: "Sag mir Bescheid" or "Gib mir Bescheid".

Another example where "lassen" does not translate to "to let" is the pattern: "etwas machen lassen", which translates to "to have something done". 
Example: "Ich lasse mir die Haare schneiden" --> "I HAVE my hair cut".

Answer

Ich bin kein Muttersprachler mit durchschnittlichem Deutsch, nach meiner Kenntnisse,  man könnte entweder "Lass mich wissen" oder "gib mir Bescheid" um etwas wie "let me know" auszudrücken.

Und hier mehr über das Thema: 
https://easy-deutsch.de/verben/lassen/

Answer

It also means to leave, in the sense of leaving something behind, is sort of a modal verb, and can be used as an alternative to the passive.

This is a good explanation:
https://yourdailygerman.com/2014/10/01/lassen-meaning-german-passiversatzform/

BTW in French it's laisser, so you can see how close the three languages are and I'd bet the other Germanic and Romance languages have a similar word beginning with L.