This was discussed previously here.

The Writers Guild of America's screen credits manual has this definition:

"Adaptation by"
In the interest of avoiding a multiplicity of credits, the Guild is opposed to the general use of the "Adaptation by" credit. However, the Guild recognizes that there are certain unusual cases where credit is due a writer who shapes the direction of screenplay construction without qualifying for "Screenplay by" credit. In those special cases, and only as a result of arbitration, the "Adaptation by" credit may be used.

So the answer is omit the credit. Hope that helps.

I always include Adaptation/Adapted by credits locally but usually don't contribute them because they're controversial and if people don't want that data it's OK.  If there is an Adapted by credit and a Screenwriter credit I believe it should (generally) not be contributed.

However, there are instances where I believe some Adaptation/Adapted by credits are legitimate screenwriting credits and would have no objection to them being contributed if documented properly in the notes.

e.g.
The Jazz Singer
    A Photo-Dramatic Production of Samson Raphaelson's Play (OMB)
    Adaptation - Alfred A. Cohn (Screenwriter)

In Old Arizona
    Based on O. Henry's "The Caballero's Way" (OMB)
    Adaptation and Dialogue by Tom Barry (Screenwriter)

Bulldog Drummond
    Based on the International Stage Success by "Sapper" (OMB)
    Adapted for the Talking Screen by Sidney Howard (Screenwriter)

Ladies of Leisure
    Adapted from a David Belasco-Milton Herbert Gropper Stage Play (OMB)
    Adaptation and Dialogue - Jo Swerling (Screenwriter)

The African Queen
    Based on the Novel "The African Queen" by C.S. Forester (OMB)
    Adapted for the Screen by James Agee & John Huston (Screenwriter)

etc.

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