Ubuntu – How to digitally sign documents in OpenOffice under Ubuntu

On Ubuntu 19.04, Libreoffice seems to be there, but when I clicked, all the apps seem to be missing (like the word processor and the spreadsheet). So I first installed OpenOffice. Then I figured out that I could install Libreoffice as well:

sudo apt install libreoffice

so I now can verify that both have the same issue.

Both Openoffice and Libreoffice have an option claiming to digitally sign a document.

So I created a certificate using instructions from this site: https://websiteforstudents.com/self-signed-certificates-ubuntu-17-04-17-10/
Here is the shell script I wrote to do it so I would not forget the steps:

#! /bin/bash
# see https://websiteforstudents.com/self-signed-certificates-ubuntu-17-04-17-10/

name=dbk
openssl genrsa -aes128 -out $name.key 2048 #generate a key
#best to generate without a passphrase, so next command removes it
openssl rsa -in $name.key -out $name.key

# create a certificate signing request. This one is good for a year
days=365
openssl req -new -days $days -key $name.key -out $name.csr

openssl x509 -in $name.csr -out $name.crt -req -signkey $name.key -days $days

#make it readable only to you, to protect it
chmod 400 $name.*

OpenOffice still didn't find the certificate or offer me any way to select a directory. In the wiki for openoffice, I found a page claiming there are 4 ways to select the directory where OpenOffice will look:
https://wiki.openoffice.org/wiki/How_to_use_digital_Signatures

One of those ways would be to set an environment variable:

export MOZILLA_CERTIFICATE_FOLDER=~/cert

This didn't work either.
In Libreoffice, I can see an option to start a certificate manager.
When I click, I get the following window.

I can click on the certificate in the cert directory but the open button is not clickable.

How can I get either word processor to find the certificate that I created in ~/cert, or is there something wrong with the steps I used to create it?