You can add budget to a campaign at any time after the campaign has already begun.
When entering the additional budget amount, please follow this formula:
Amount Initially Added + Additional Budget Amount = New Total Budget
For example, if you had a campaign with an initial budget of $1000 and you would like to add an additional $500, the new total budget would be $1,500.
Even if any amount of the initial budget was already spent, you are still required to type in the full amount when adding additional funds to a campaign.
Choosing to add amount will open this window:
Add the budget and confirm the addition.
There are quicker ways to add budget to a campaign. Read more about it