You can assign a Primary Contact to your Client record.
Navigate to the Client record to which you wish to add a Contact.
Search for the relevant contact within the Primary Contact field:
If the Contact does not exist, click '+' to add a New Contact
A quick create Contact form will open:
When creating a Contact for a Client, make sure that the Account name field is assigned to the relevant Client.
Complete the mandatory fields and click Save.
Once saved, your Contact record will appear as the Primary Contact against your Client record.