How to create custom drawing symbol

This video should get you started.

https://www.youtube.com/watch?v=TZLOIb-dUp8 

PTC Learning connector if you have access:

https://learningconnector.ptc.com/tutorial/1625635536001/create-symbols 

Here is a lengthy thread about custom weld symbols. From earlier versions, so not sure what has changed:

https://community.ptc.com/t5/3D-Part-Assembly-Design/Weld-Note/m-p/299415