Cannot save javascript code in head and body options

Hello,
I explain my problem: once the plugin is installed, I cannot paste a <script> tag in the “head” and “body” fields. If I save this causes me a 403 error. On the other hand, if I save any character string without the <script> tag, like an html tag, it will pass …. Do you have any ideas to help me please? I specify that I have no particular rule in the .htaccess and that the problem arises as well when the server and software firewalls are active or inactive … Thank you in advance.
Regards.