Steps to reproduce:

• Open any trigger prototype in any discovery rule.
• fill any {#SAMPLE_MACRO} in the URL field
• click on update

Result:
ERROR: Wrong value for url field.

Expected:
Should be updated.
In Doc this field is supported https://www.zabbix.com/documentation/3.4/manual/config/macros/lld_macros

LLD macros can be used:
....
for trigger prototypes in:

• names
• expressions
• URLs (supported since 3.0.0)
• descriptions (supported since 2.2.0)
• event tag names and values (supported since 3.2.0)
  ...