What data you would like to show under the Custom View form?
If the Lookup field is a mult-value enabled field, then you would need a ListBox or a Dropdown (Combo Box) control to show the values.
The function to show all the available values for the Lookup field is:
Choices(SharePointList.ChoiceFieldName)
If you would like to filter the value:
formula should be:
Filter(Choices(SharePointList.ChoiceFieldName), Condition expression)
Regards,
Michael