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Their is no need to amend the soil before planting forsythia. It is naturally vigorous without special care. Studies show that a planting hole with rich soil creates a type of barrier to roots trying to extend out to the unimproved soil. It is as if the roots are spoiled by the enrichment and refuse to go past it. If you wish, apply 2 inches of compost after planting. Do remove 1/3 of the branches to the ground every year after right after your forsythia blooms. Start with the oldest, largest branches. This method keeps young wood emerging. This young wood blooms more heavily than older wood and has more leaves. Do not prune forsythia in the fall as you will be cutting off next year's blooms; they form over summer. Start your pruning regimen next year.

This is an alternative to a raised bed. The plant roots receive more needed oxygen. It is easy to cultivate or hoe weeds since it can be done standing. Even gardeners who don't use this technique for most of their garden may make hills for growing squash family, cucumbers and corn. This is a variation with similar advantages to mounded rows. Mounding can also be a work around for people with heavy clay soil. The mounds do dry out faster than level ground so it may not be a good strategy for those who live in hot, dry and/or windy sites.

You can easily get a soil test kit and your soil test done by your County Extension Office. They charge just a very small amount for this test.
This link will help you locate the nearest office.
https://www.gardeningknowhow.com/extension-search

There are several things that you can do here. I recommend either superphosphoric acid, or citric acid in small amounts until you get where you want to be.

You could also apply sulfur to acidify the soil. I prefer wettable sulfur, since it is fast acting.

This article will give you more information on the subject: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/raise-acid-level-soil.htm

Contact a local landscape company.
Check with your County Extension service; they may have a reference for you.
https://www.gardeningknowhow.com/extension-search