UPDATE:
Because the mention of Gesis rung a bell, I found a link to a better introduction to what I think you are using(?). See https://notebooks.gesis.org/ . There it says your repo should be ‘Binder-ready’. That means you’ll want a configuration file in there called requirements.txt and the contents of the file requirements.txt would be the following for the packages you listed in your original post:
pandas
numpy
Then with that requirements.txt file in your repository, when it starts up the session you’ll have those two packages installed.
If you absolutely need to install some other items using conda, you can move to using an environment.yml file instead of requirements.txt. environment.yml allows specifying both conda- and pip-installable items. See here and here for more about that.
What I originally wrote in a draft of my reply is found below and may be worth a read because I think it provides more background that might help you better understand what is going on or offer you options:
I have used Gesis some as they are in the binder federation. However, only really via Binder where it is clear you include a requirements.txt file that would include pandas and numpy listed and then when the session starts they are already installed. See here and here.
Those packages you listed are at the PyPi repository so you can simply use the pip package manager to install them, which is what requirements.txt uses.
Alternatively, you can put the following lines as a cell early in your notebook if you don’t want to bother with environment configuration settings:
%pip install pandas
%pip install numpy
Note the use of % at the start. That magic works for conda, too, and is now the current ‘best practice’ preferred to do installing when you are in a notebook. See here.