Startup Manager

Drive Partition and Format - El Capitan or Later

1. Restart the computer. At or before the chime press and hold down the Command-R keys until the Utility Menu appears.
2. Select Disk Utility from the menu and click on the Continue button.
3. After Disk Utility loads select the volume (indented entry) from the side list. 
4. Click on the Partition tab in the Disk Utility toolbar. A panel should open with the Partition Circle.
5. In the Partition Circle click on the Add [+] button below to add a new volume. Click in the new volume. It turn blue. Use the fields on the right to set the size.
6. Click on the Apply button and click on the Done button when it is activated.
7. Quit Disk Utility.
8. Restart the computer.

If you have an APFS formatted SSD, then after you click on the Partition tab a panel will appear in which you can select a standard partition option or add a new volume to the existing Container. Click on the Container button and proceed as in the above.

If you mean "Command + R" to enter Recovery Mode that is something else. You can't partition a drive using the bootable installer.

Boot Using OPTION key

1. Restart the computer.
2. Immediately after the chime press and hold down the  "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon from which you want to boot.
5. Click on the arrow button under the drive icon.