It is easier than you know. Two ways that I know of. The easiest is the handcuffs just above text entry
If you click on it this is what happens
In the paste field type in the name of the thread you want to link. I entered a thread I created about SOG
Another Clone SOG and TBD
The other way is more cumbersome and what I did to enter your thread up above.
I went to your home page / activity / topics
I highlighted the topic and used copy command
returned to the page I started from (back key) then pasted the address of the copied thread.
I should mention this is all on my laptop. NO F’n clue how to any of this using a phone.