Use a greater-than sign (>) to split long lines, without interior line breaks:

    - set_fact:
        msg: >
          This is the first part
          and second part of the string

    - debug:
        var: msg

Results in:

"msg": "This is the first part and second part of the string\n"

Use a pipe sign (|) to split long lines, preserving interior line breaks:

    - set_fact:
        msg: |
          This is the first part
          and second part of the string

    - debug:
        var: msg

Results in:

"msg": "This is the first part \nand second part of the string\n"

Add a minus sign (-) after a greater-than or pipe symbol to remove the line break at the end of the line (use >- or |-)

See: https://stackoverflow.com/questions/3790454/how-do-i-break-a-string-in-yaml-over-multiple-lines