Hi ,
Could you share more details and screenshots? Where should the combo box be? in EditForm or gallery or standalone?
I assume it is standalone.
To get your requirements, we have to change another workaround. Retrieve the Contact list in combobox directly instead of using Choices function.
1. Add a combo box, set Items property to 'Contact list'
2. Set Field layout to Double, and modify the Primary text and Secondary text to display the company Name AND the full Name
3. Set SearchFields property to ["surname","Company"], then you can search by two columns
4. Add a EidtForm, set Items property: ComboBox1.Selected.
5. Add a button, Set OnSelect property: SubmitForm(Form1);Patch(Events,First(Events),{ContactID:{'@odata.type':"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedReference",Id: ComboBox1.Selected.ID,Value: ComboBox1.Selected.<Contact>}}) //<Contact> is the column ContactID lookupAfter all, it can select the contact, edit the detail form, bind the selected contact to the first event record. If you want to change to bind to other records, just replace the First(Events) part.
To your third question, you have to modify the DefaultSelectedItems property of combobox. What i can say is that you have to put a record form the 'Contact list' in this property, which means it should have the same table structure with combo box item.
So for example, if you want the combo box to display that default contact of first event record, you can apply LookUp('Contact list', ID=First('Test-1009').OID.Id) to DefaultSelectedItems property.
Sik

@TheLuckyOne