I'm not sure if I understand what you mean but you have to add the lexer into
https://github.com/geany/geany/blob/71969f600023a333bcd3b717a434082a7039f316...
This is already included in this PR since last year. ^^
https://github.com/geany/geany/pull/3480/files#diff-30d8f6be6320feeacf686be9...