Very easy.

Put your finger on what you want and hold. A drop down will appear with various things to do. Hit copy link, and share link. It will say at the bottom, copied. Go to wherever you want to put it, like here in the group, etc and hold your finger on the blank message post and it will copy the url.

Good luck!

Also if its just a written message just copy and paste and take it where you want it posted.

I'm not good at explaining things so I hope you understand.