hello
try this
simply split and concatenate the strings then save the result
for me with writematrix as I run R2020, with writelines if you are equipped with R2022
tmp1 = readlines('Al.txt');
tmp1(strcmp(tmp1,"")) = []; % removes empty lines (needed ?)
tmp2 = readlines('TestFile1.txt');
line_insertion = 6; % define at which line of A1 text you insert TestFile1 text
out = [tmp2(1:line_insertion-1); tmp1; tmp2(line_insertion+1:end)];
writelines(out, 'Output.txt')
% writematrix(out, 'Output.txt') % before R2022
