Create and populate attribute for 20 datasets?

Create an Excel file with the filename and Trajectnaam and join that to your data using a DatabaseJoiner or FeatureMerger.

Input format is a ESRI file gdb, I will try your suggestion.

If you are reading from a single gdb, and you want the featureclass name to be populated into Trajectnaam you will need to expose the fme_feature_type, not the dataset. If you add the gdb and choose to add a single merged feature type this will be exposed automatically

Could you maybe post a screenshot where I can find the fme_feature_type?

This seems to be a bit time consuming, because my dataset names are already in my workbench. The only step I need to do is to acces them and connect a Trajectnaam to them.

You could also try an AttributeValueMapper and have that import the unique values from a source dataset. Or... use FME to generate a list and save that in Excel, then edit that Excel file to suit your needs.

If you click on the cog, then go into format attributes you can select it here

You could also just add an attributeexposer and type in fme_feature_type

@lambertus In addition to using the fme_feature_type to set your Trajectnaam attribute, you can probably simplify your workspace by using a dynamic workflow pattern in workbench.

With your current configuration, if your source File geodb changes, i.e. a new feature class is added, then you'll have to update your workspace. If this isn't likely to occur, then you don't need to read any further. However, if the source data might change then you can look at using the dynamic workflow patterns in FME. There's a tutorial on this if you want to explore it further: Tutorial: Dynamic Workflows

You might also benefit from a dynamic workflow pattern on your output.

Ah, thanks! I found a solution in the meantime. But still this is useful info :)