First, there’s an easier way, which works in both eevee and cycles.
1. Select Plane
2. Properties editor->Object tab->Visibility tab->[eevee]uncheck shadows under ray visibility. [Cycles] Ray Visibly subpanel->Uncheck Shadows
Second, The answer to your question:
1. Select Light
2. Properties editor->Object tab->Shading Panel->Shadow Linking subpannel->Click New
3. Add the Plane to the Shadow Linking by Pressing and holding LMB on the plane in the outliner add drag and drop it in the shodow linking panel.
4. Uncheck Plane.
