Hi Rosana,
Based on your description:
What type of field is this custom field? You want to click it directly into the associated contact record, meaning it must be a lookup field.
If so, you could have just used the 'Existing Contact' field, after all, updating a Qualified Lead is quite a hassle.
We need to do a little customization in order to fit the business needs:
1.Remove 'Existing Contact' field from the BPF.
2.Add this field to the lead form
--'Parent Contact for lead' is the display name of the field, you can change it on the form by changing the label:
3.Show/hide this field based on field status, male sure that it can only be shown on the qualified lead.
Test Result:
--Qualified Lead:
--Not Qualified:
I hope you can mark my answer verified if it answer your question! If you have any questions, please feel free to contact me.
Regards,
Leah