If the product you are using is the Garden Basics 2L Hose On Pack Liquid Fertiliser Lawn Food, it is an organic fertiliser and can be applied whenever you want. You might like to avoid applying it before heavy rain as the majority could be flushed away. If you are using chemical fertiliser, please refer to the directions for use on the packaging. This will normally state if it can be applied before the rain.
If there is a specific product you're after the information about, please feel free to post the link, and I'll be happy to contact the manufacturer for you.