How to restrict "Hire an Employee" without adding a Line Manager in Assign Manager

Summary:

Step1: Login as HR Specialist

Step2: My Client Group -> (Quick Action)Hire an Employee

Step 3: Enter details into "When and Why", "Personal Info", "Employment Info"

Step 4: Do not add "Manager Details" into "Assign Manager"

Step 5: Submit

Content (required):

Actual Result: A worker can be hired without having a line manager assigned in the above ways (a & b)

Expected Result: Colleague should have line manager assigned while hiring.

Version (include the version you are using, if applicable):

21C, 21D

Code Snippet (add any code snippets that support your topic, if applicable):