SOLVED — French poetry ...tirêt ? —

Thank you Romanos and Marc for your answer.
How do you insert U+ codes ?
I agree underscore isn’t beautiful (its typographically an error to use it ; historically it was an underline in the tipemachine Du bon usage des tirets | Bruno Bernard Typographie)
Isn’t possible to insert a curved under-ligature ?

The curved under ligature is exactly what you get when you use underscore.

Thank you Marc. I’m sorry if I’ve embarrassed you for nothing. I had not the idea to try it.