If you’re on a computer, select and copy the link by highlighting all of the link in the address bar on YouTube, and use the right mouse button to click ‘copy’. Then on Tattle, in the space where you write posts, press right click again and click ‘paste’ and it should copy the link over.
If on a phone, do the bit above in my first reply, then on Tattle in the space where you write posts, if you press the screen and hold your finger on it, an option should come up to ‘paste’. Sorry, I’m probably making it sound complicated, if still need help I will try and do screenshots