If your creating a project but need a particular Coordinate System for that project, you can create these from the interface below. Before adding a new system, details of the system are required, can be found by doing a search on the 2 URL's below
https://www.spatialreference.org/
Add the CRS value, this will be the ESPG number. Example "EPSG:26919"
For the name, add the name of the system you searched for. Example "NAD83 / UTM zone 19N"
In the Proj box add the PROJ.4 line of data you copied from the site.
Example:+proj=utm +zone=19 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
Also then tick the "is Active" option and save the changes.
The system will then be ready to use in a project.