Donorbox creates Salesforce campaigns based on the name of your campaign on Donorbox. It does this only if it cannot find a campaign with the exact same name in your Salesforce org. If the campaign is already present in Salesforce, Donorbox would use that. In order for data on a Donorbox campaign to be routed to a specific campaign in Salesforce, their names must be the same.
You have created a campaign on Donorbox called 'Help Save the Rainforest'. You then create a campaign on Salesforce called 'Help Save the Rainforest'. The data from your 'Help Save the Rainforest' campaign on Donorbox will then be allocated to your 'Help Save the Rainforest' campaign on Salesforce because the two campaigns share a name. Similarly you can create the campaign in Salesforce first and in Donorbox afterwards and the results would be the same.
In short, it doesn't matter on which platform the campaign was created first. What matters is the fact that data from a Donorbox campaign can only be routed to a specific Salesforce campaign if the names are an exact (case-sensitive) match. The below images show how a campaign in donorbox was matched to a campaign in Salesforce:
Donorbox campaign name
Salesforce campaign name