Hi,
You can define that in your request. Suppose there is already a contact then set Create Contact to false and pass the contactid - something like this in the body:
POST {{webapiurl}}/leads(E8079BFE-1AB1-449D-961D-94C32B690361)/Microsoft.Dynamics.CRM.QualifyLead
BODY:
{
"CreateAccount":false,
"CreateContact":false,
"CreateOpportunity":true,
"OpportunityCustomerId":{
"@odata.type":"Microsoft.Dynamics.CRM.contact",
"contactid":"C9BF9A01-B056-E711-ABAA-00155D701C02"
},
"Status":-1,
"SuppressDuplicateDetection":true
}