Hello,
You need to use MethodAPIUpdateV2 to set IsLeadStatusOnly to false where the RecordID matches the lead you want to convert into a customer. Here’s some sample c# code that does this:
var MAPIsvc = new MethodAPIService.Service();
string account = txtAccountName.Text;
string email = txtLoginEmail.Text;
string pwd = txtLoginPassword.Text;
string RecordID_To_Update = "446";
string tablename = "Customer";
string[ updatefields = "IsLeadStatusOnly".Split(',');
string[ updatevalues = "false".Split(',');
txtResponse.Text = MAPIsvc.MethodAPIUpdateV2(account, email, pwd, "", tablename, updatefields, updatevalues,RecordID_To_Update);