Using if/then branching logic in workflows, you can set a "default" property value for contacts who meet the enrollment criteria of the workflow, but don't already have a value set for that particular property.
In your HubSpot account, navigate to Automation > Workflows.
Click the name of the workflow you want to edit.
Click the plusicon+.
In the right panel, select If/then branch.
Select Contact propertyas your if/then branch criteria.
Click the dropdown menu and select theproperty your want to set the default value for, then select is known as your criteria option.
Next, under the No branch, click the plus icon +.
In the right panel, select Set contact property value.
Select the property you want to set a default for (the same one you used in the if/then branch criteria) and select the value you want to set for all contacts who do not currently have a value for this property.
Click Save.
You can let the Yes branch end (don't add actions), unless you have other actions for contacts who go down the Yes branch.
Once enrolled in this workflow, the contact's empty property will update with the default value you set in the workflow.
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