I have a requirement that in Filed client_Logo__c(URL) whenever user enters any URL : IF it dosen't begin with http or https then it must throw a validation error.
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For your case, if this is a default field, create a
Validation Rule on your object.
Error Condition Formula
NOT( OR( BEGINS( client_Logo__c, "http"), BEGINS( client_Logo__c, "https"), ISBLANK( client_Logo__c ) ) )
The rest (Rule Name, Error message) can be whatever you like
Edited: Updated formula for further requirements
Have a look at the following