Use ‘Custom event condition’ as the condition to create a check based on the event property of the event selected in the trigger - Occurrence of an event. 

For example - Use this condition with the trigger on occurrence of a sign up event to qualify contacts on the basis of Contact Source event property (Sign up from Mobile, Sign up on Desktop etc). 

Key steps -

Step 1: Drag and drop ‘Custom event condition’ card from the left panel. 

Step 2: Connect it to the desired card in the journey. 

Step 3: Click on the ‘Custom event condition’ card to open the right panel. 

Step 4: Select the event property from the dropdown. 

Step 5: Select the operator and add the event property value in the input field.

Step 6:  Click on the ‘Save’ button to complete the configuration. 

This would mean, whenever contact reaches the ‘Custom event condition’, it would check for the event property value equal to the value entered in the journey and qualify contacts into YES and NO routes in the journey.