Mandatory Line Manager field

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I have a requirement to make the Line Manager field mandatory. 

I can see that via a personalisation I can make the Name field mandatory; and i can make the Type field mandatory; but how do i ensure that a Line Manager value is entered and that the user could not enter a name and select a different manager type e.g. dotted-line manager as this would meet the mandatory requirement and allow the action to be saved - but the Line Manager value would still be missing.

Also, anything we should consider before making the field mandatory?

 

Thank you