There are several ways:
Select the client from the list in the Clients tab, go to the Summary, find the Recent contacts section and click the red circle at the bottom right:
2. Select the client from the list in the 'CRM'> 'Clients' tab, go to the Sales activities and click the red circle at the bottom right:
3. Go to the CRM-->Sales activities tab and click on the blue Add activity button visible in the right, upper corner of the page:
Fill in the fields, select the client from the list and click Save:
4. Select the client from the list in the Clients tab, go to the Summary and find the Contact people section, click the three dots by the name and select 'Add activity'.