Leads or deals are almost always linked to a contact or a company. Use the standard Client field in CRM entity forms to set this link up.
Open a lead or deal in a browser or the Desktop app.
If you don't have this field in the CRM entity form, that means it's been hidden. Click on the Select field button and find it in the list.
You can find the existing contact or company. Just start entering the text into the Client field. The search is done by name, phone number and email address of the contact or company.
If nothing is found, you can create a new contact or company in one click.
If you have created a contact and a company in a deal form at the same time, Bitrix24 will automatically link them.
You can configure this field by clicking the gear button. Hide specific details, select what to show first, or disable quick edit.