Oh, well as far as I know there are no event befor the user have placed the request.
Could maybe add a Business Rules to hide a mandatory field or change the option for a dropdown field that say Error 

If field = X 
Set mandatory Y field 
Hide Y field 
Remove option for a drop down to only show error


Not sure if it’s going to work and it’s not pretty.  
 

I could try this as a workaround.

Hello, 

You can use the new expression function the condition block.
On this page  Expression builder Node : Freshservice you see this  ‘{{ticket.actual_requester.reporting_manager.name}}’== ‘{{ticket.requester.name}}’

That will give you a true or false. Down (No) you set a  not or a tag to indicate a error and right (yes) you keep continue your work flow. 

Reading your post again,  You can add a note to the ticket or a tag. 

Thank you Daniel,

but I would like to stop the user from placing the request.

When the following condition of the workfow evaluates to “True’ then I would like to stop the user from placing the request. The Action of the workfow does not have an option.

abs({{ticket.ri_55_cf_new_request_week_no}}-{{ticket.ri_55_cf_request_week}})>8

Hi @kostaskourtides - have you looked at Business Rules for Service Requests?

They have the option to disallow form submission.

You can use relative dates based on your date field as here - I think this is what you were aiming at?  Not allowing an end date of less than 8 weeks?  But whatever (lol) you can edit the Condition to get the result you are wanting to see.

HTH
Bryn at CYDEF.ca