Hi Farah! Currently, there are no out of the box methods to obtain pixel values in Web AppBuilder. However, here are a few workarounds that have seemed to work for users in the past.
1) Using a custom script. Two that you can adapt/reference include https://jsbin.com/vazadajoyi/edit?html,output and http://jsfiddle.net/kevinmSAGIS/L834u2nt/4/. Please note that these are not official Esri resources, so they are for you to use at your own discretion
2) You can also utilize Esri's World Elevation/Terrain, but please note that this is premium content, so it does expend credits. You should be able to enable pop-ups containing the pixel value information with this.
3) You can use the Raster to Polygon tool or Raster to Point tool to convert rasters into feature classes and add that to the feature service.
4) You can publish the raster as an image service as opposed to a Map Service. This is not a capability with ArcGIS Online so you will need ArcGIS Server for this. Feature access capability is available for a feature class, but not a raster.
There is currently an enhancement to have this functionality. There is also some discussion about workarounds as well on this page: https://community.esri.com/ideas/12329. Feel free to vote on it, as this is the best way to get the developers attention.
