To search based on custom objects and/or fields, you will need to use the Search Object Actions.
Using this action, you can search based on any field inside your object by selecting those fields in the optional field list. At least one of the search fields should be specified.
Below is an example of an email search field being specified.
On the other hand, not specifying any field inside the search will throw an error. This is because at least one field is required for the action search to be completed.
Selecting multiple fields is also possible. This will make the search action an "AND" condition. Fields selected will have to be populated like in the picture below. In this example, only contacts that match the given first name, last name and email address will be retrieved.
If a search action needs to be in an "OR" condition, an additional search action step can be added. Below is a sample of the step procedure.
Action on step 1 is searching for the Contacts in Salesforce with only the email address.
Whilst on Step 2, First name AND Last name needs to be present to return a valid search.
Note: If any new fields have been added to your application, a refresh schema is a must. This is so the new fields will get picked up.