Hello everyone, I’m new to this fantastic world of icinga2 !!!
I come immediately to my first question, when I create a custom command via the icinga director web interface, in the arguments tab I see custom variables, but I can’t find where to go to set them
Thanks everyone for the help
Hi and welcome.
I edited your post so it shows the screenshot correctly.
Tbh this does not look like the Icinga Director (reminds me more of Centreon).
To use and fill custom variables you need to create a service template (and e.g. a service apply rule after that) which uses the created command. There you can add the variable fields and the fill them with the information needed.
Thank you so much for your help, and for the advice! I will look at the previous posts to understand how to better format future posts! thanks and good job
Maybe check the FAQ for formattings tips: https://community.icinga.com/faq#format-markdown 
Images can simply be copy-pasted(ctrl-v) into the post editor.
For learing the basic logic behind plugins, commands and services I suggest you check the documentation:
https://icinga.com/docs/icinga2/latest/doc/05-service-monitoring/
After this here is a good tutorial on adding a host and service with the Director: